This game is awesome! I'm glad you are making a second one. If you're still accepting ideas, I do have a few.
* Have multiple classes/skill trees. I'd like to build my character in a specific way that won't be the same as everyone else's at the end of the game; to be able to focus on magic/ranged/physical or even have a beast tamer thing or something would just be neat.
* A storyline! Have it continue on where this game was really just the 'tutorial' of sorts for your soul, where you get passed the Arena and go on an actual adventure.
Different zones would have so many battles in them, where you could choose normal hard or insane difficulty on them and get different loot.
*Better loot options. Not just a direct upgrade, but there should be choices, too. Like have some weapons with different effects, and an equipment screen.
* Allow a hit or something or not make the max combo thing so insanely high on the minigames for stat raising. I would like to get the achievements for this game, but there's no way that I am going to waste the time perfecting all of the combos for nothing. I beat the Scarecrow King at like level 110. There's no need to keep going with the combos, and the survival has no purpose beyond opening the 'Final Battle'.
* Either take away the raising of stats entirely via level, or balance the way the training minigame works and leveling stats work. I maxed out my Crit and Dodge so early, I didn't need to train in the minigame at all for them. It's useless to have the minigames when they're outshined by basic leveling. All they do is raise HP when you cap your crit ratio and stuff, so you might as well just do defense training since you can raise def and hp that way. It may be a good idea just to raise stats through the mini game.
* Make more types of minigames! Have harder versions with different controls open up as you master the previous ones, that will raise stats better. That, or just random cycling between them for more variety. Bring in new mechanics or things to spice up the current ones!
* The pet system you mentioned would be awesome.
* More secrets!!!! And ones that aren't listed in the secrets page thingy.
* Make the income generator more useful, or just take it out entirely. I maxed out the income thing at the bank first and foremost whenever I had money. It literally was never useful to me. Granted, I beat the game in 3 days. So it may be of use to someone who doesn't time-dump in it hahaha. But it still didn't take very many hours from me. It was just useless and costed too much to raise and didn't give enough back.
By having more content to last longer, making the bank cheaper and giving you some more money while also lowering the money you get from battles, this feature could be quite useful. Maybe even quests you do for the banker or a minigame to raise the income level you get? Also, it should max at like 75%. The banker wouldn't want to give all his money to you lol.
* Do more with the basic attacks. I really liked the dodging mechanic in the dodge minigame. Maybe have different weapons or something have normal timed button presses, or blocks also be timed(with a natural % too) or give the choice to hit low med high or something for more variety.
* balance the potion system. The yellow potion for lowering the timer was just insanely overpowered, where at the max cooldown reducer skill and using every skill and instantly using the potion, it would reset every single one for me.
* Do more with the housing system. Give options for the type of house you can build and where you place stuff, and make it so you can decorate inside of it! Maybe have a 'character' page thing where you can link your house and how it looks or something.
* Make an insanely high-leveled boss for people who want to spend a ton of time with this. lol. Add content enough to give more hours into this as well. Even if it's light content and not terribly serious; giving more zones and fleshing out the world you've put forward in that map would be pretty sweet.
You could take something that had little story and build on to it to be something quite great. This is a really fun game, but it feels like there's just quite a bit missing that, if included, would make this an amazing game.
Good job!