My question is what should be in a Serious, Pro, Multiplayer FPS?
This isn't what should NOT be in one, its what should be in one.

GUNS

But seriously, I've actually never played one (unless you count that time I played Time Splitters 3 at a friends house, but I'm assuming you meant online multiplayer). Capture that Flag is fun in real life, so I guess it could be fun with guns and explosions and stuff. Yeah I dunno.

But seriously, I've actually never played one

Really? I guess your young? because I would be so bored without fps games I love them.

And a game I believe you need to be pro at to do well is Counter strike source purely because spraying won't get you kills that often you need to control you shots and make them count because you can die within 1 shot to the head with most weapons so you have to be precise and have good reflexes.

I'm human and I can't read most of these captchas its so annoying.

My question is what should be in a Serious, Pro, Multiplayer FPS?
This isn't what should NOT be in one, its what should be in one.

Do you mean realistic or what because I am sure there is already a game out there that fits your description.

Like realistic would be something like one or 2 shots kills someone and they start to bleed out. But really no one wants a realistic game. A human just can't survive from that much type of pain. So if it was realistic people would need to eat and drink, have fatigue need sleep. Have to have muscle to carry certain things. Weapons that hit you in the leg and you can't walk.

Really any game can be serious and pro. It is more common to find this in a PC game because people create servers and mods that make the game to what they want. But then there are groups like MLG that really seem to define what would be Hardcore so to say.

What should be in a serious, professional and multiplayer FPS game?
Balance

Variation in strategy -- knowing the textbook actions and reactions is nothing in professional play, it's just the players literally knowing or executing the actions better than the other team. Have it so that there are different traits each player -- and each team has that make them excel at some things and perhaps not so much on others.

A sturdy metagame -- have it so that strategies chosen on a map and side can be countered by the other before it's too late. This can be very difficult but it's what I think ought to be in an FPS game played seriously. This is especially useful if you include the previous case -- where people will all play in a Rusher, Boomer or Turtle fashion (those are RTS terms primarily but still) because they would be deemed the most effective. Instead have one team's Boomer be  better than another team's Turtle, because of skill variation -- where there could be a Rusher that beats the Boomer but due to the choice of strategy, Turtle would have a chance against Rusher.

A Mod or mode made for professional play -- anything else would be far too bland for your common player, and you need to make money from sales so have something in the game that can at least cater to some more casual individuals, but not stripping away from the professional play that hopefully is the core game.
Or you could have a little variation in the game but ultimately have it prove less effective than other options against pros -- that way you can still have some strength with items against lower skilled players and maintain a fun factor.

PC professional play -- this is quite obvious honestly. The PC is objectively better than consoles at FPS play when it comes down to control. If you're going to have a game played professionally then have the decency of not limiting the players by the control method they play on -- the PC is the best known choice thus far.

With the computer I am brought to two more points for an FPS game:
1) Dedicated Servers -- this is basic, and present in all PC titles (that can be called 'PC titles'). Allow this for community, ease with mods (the next point) and having server options so that FPS players can pick their options for what they want present in the game (perhaps the Pro Play 'mode' is not a mode at all but rather a specification on what is and isn't allowed in the server).
2) Mod Editor and mods on Dedicated Servers -- mods can improve the longevity of any game prodigiously, and it's also been present on many FPS titles for the PC (including Call of Duty 4 and Battlefield 2). This, like Dedicated Servers is essential, for that very reason of making a huge variance in what the game has to offer and doing so for free once the editor has been introduced.

- H

dead ends in maps are a fun breaker. make sure there are always atleast 2 way's some1 can go.

also make sure that all players have the same powers. so that it comes to the amount of skill the player has. and not the perks he bought/unlocked.

Here's what i was thinking so far, please give feedback if it sounds good or why omething would not work.
I would have classes for the game (so that player who are more tune to a certain type of playig won't have a disadvantage)Basicly have a stealth (all accuracy, short supply of ammo, blending capibilities with the map, also I was thinking of implementing a feature where you could go up to enimeis and add something that would screw with the accuracy of their gun's accuracy for a short time) because of its stealth capabilities this class will specialize in close range combat and close range traps (trip mines and regular step mines) Then I would have a real spray and pray class (but this one will be just for noobs since any other class will have an extream advantage over it if the attacking player is good enough) just giving them a heavy machiene gun which has such low accuracy that at mid range its medium but could be used like a shotgun at close range since it spits so many bullets. Also they will be equipted with a knife (as well as all other classes) and a pistol which could be used more effectivley than the spray and pray gun, requires you to aim somewhat (get them used to accuracy) Then there will be an assualt class (refined spray and pray) which will have an single shot assualt rifle which is effective at medium range and could somewhat do far, not too good at close range though snce its only single shot.  The pistol would be loud and not shoot as fast as the assualt rifle, but would be more effective at killing at close range. Then there is the sniper class, incredebly bad at close range (no fancy no scoping here) where you will not be able to shoot from the hip and will have to resort to a quite weak machiene pistol instead, the sniper rifle will be really effective at long range, have really high zooming capibilities (but will be really hard to control the farther you go, lying down will reduce the sway) You could also (as a tribute to real snipers) gather up surrounding shrub and stuff and put them on your character so you blend into the enviroment the noise of the sniper will be quite loud on full power, or at 50%  reduced power you could have the sniper rifle half as loud.

Im deciding between two very different things and I need you guys to help me to decide, have no bull**** just green squares as the enimies (so then the hitboxes wouldn't be all screwed up) or just do it like everyone else and put normal looking, weird hitbox machanics, humans.

Also I was thinking of having a bleeding out system where there would actually be a trail of blood dipiing onto the floor from ur character and life leeched as well which could only be fixed using bandages which you have to get from spawning or from dead bodies on the floor (if they still have some)

Do you mean realistic or what because I am sure there is already a game out there that fits your description.

I'm looking for one that people could go on to prove that they're a btr fps gamer than the other person.

having server options so that FPS players can pick their options for what they want present in the game (perhaps the Pro Play 'mode' is not a mode at all but rather a specification on what is and isn't allowed in the server).

I think that would be a good idea since there might be things that other people might not want, I was thinking of having boxes or real players a toggleable thing so that players who don't want to guess if they hit in the hitbox, could indeed know.

doing so for free once the editor has been introduced.

Definatley sounds like an interesting idea, also would help me alot so I won't have to add alot of features for everything. I'm thinking of having it like the ETQW way of modding (or TF2) where the player just downloads the mod packages along with joining the server.

dead ends in maps are a fun breaker. make sure there are always atleast 2 way's some1 can go.

Indeed room camping is always a problem in games, having lumtiple enterences/exits help eliminate that problem.

also make sure that all players have the same powers. so that it comes to the amount of skill the player has. and not the perks he bought/unlocked.

I was thinking of having lvls do nothing except show how much you've played since it's always unfair when high lvls have better stuff (especially in SSL)

Really? I guess your young? because I would be so bored without fps games I love them.

Yeah, I guess I could be considered young.

Should any of the guns have a spray capability or no? Since not having one would force the player to aim but it might frusterate others, what do you think?

Try both in actual playtesting, then pick the one that feels more fun to use. Sometimes, those kinds of decisions aren't really all that simple to make in theory, methinks.

Should any of the guns have a spray capability or no? Since not having one would force the player to aim but it might frusterate others, what do you think?

if you have a max amount of ammo, yes.
if you can reload all the time, no.
if you can pickup so much ammo that you can reaload nearly all the time, no.