tl;dr
I'm just going to use my suggestions now.
Reward system with the battle system with stages, levels, and bosses.
Users are able to battle monsters throughout their respective tier and, as they defeat monsters, they are awarded with a unique item related to the creature (can be turned in for tu or used in the Cooking Basin to make a weapon of some sort when combined properly) and they get tu as well.
They can rebattle the monsters as many times as they like (this will mean that the tu prize will have to be substantially low for the weaker monsters) and gain experience. Experience can be used to increase abilities (EXP will have to get tougher to earn. Say, after a certain number of battles with the same monster, there's no more experience to be gained because there's nothing left to gain from battling the same thing countless times).
In order to gain a level, you'd have to fight and defeat the boss monster of your Creatu's level. Once the boss is defeated, the user gets a substantial reward and raises the level.
ONCE THE LEVEL IS RAISED THERE IS NO GOING BACK.This continues on. I understand you will be bringing back the training and, may I suggest that we use that for the only way of increasing strength, defence, and speed? Abilities are things that should be gained through experience in battle. It makes a lot of sense.
I would be enthralled if weapons required a certain restriction on them. Say, you can't use the strongest weapon in the game unless your Creatu's strength and intelligence is a certain level. Can't make it too easy. (:
PVPI think that the rewards should be larger than normal monster battles if you win a PVP battle. Yes, there should be tu rewards. Gives incentive to use the feature.
The higher the level, the more the payout.
Weapons should have their own unique ability that is randomly activated in battle.
Special weapons being ones obtained through the CS or seasonal events or random events.
Like if my RSTU001 slices through your Berrok with a Scrian Sword then there should be a chance of an extra air damage.
More to come later when I think of it.