I think the multiplayer should be like Diablo 2, where (up to 6) players could join each other in a room. I think the particles would be too much, so just show the enemy hp of ones that have been attacked recently (like in GemCraft) rather than the other player's shots.
I think the chatbox would be much more likely. I've seen more than I've expected have those.
What I want to see in BT3(?) mostly is a story/plot. After that, why not have a mode where you have to switch colors of the bubbles to hit certain enemies? Why aren't there other ships that can go to other bubbles? How about a quest mode? How about an interactive map? What about a survival mode where you can't leave the bubble?
And yes yes, the typical "more enemy types", "more ship types", "more weapon types", "more worlds" requests too.
I love the game! I want another one. More upgrades and stuff is really my only thing. I like being rewarded for going far in the game. idk. That might sound weird.
Hmm... Well the game doesn't exactly need a story... But I agree with guaplei. There could be a mode where it plays like normally, only the upgrades are more specific. For example, when you level up, you could choose to upgrade two aspects of your tank. How is this different than what's in BT2? Well, the upgrades would not be fixed (as is in BT2, if you choose the first tank upgrade, you get [insert gun upgrade here] and [insert secondary weapon upgrade here] and [insert description (albeit stale) of speed and armor]). When you level up, you could choose from a list of upgrades (everything from size of the bullets fired, to being able to reduce the number of bubbles you lose when hit) and essentially make your own custom tank. That could be, as stated, a specific game mode that could be available along with the option to play it as BT2 plays.