I kind of get tired of P&D after a few days though, also I don't like their fusion system, I'd rather my dragons gain exp from battling like pokemon or a normal rpg
I think I feel the same way about the fusion system. I find the UI quickly becomes packed with so many monsters it's hard to keep track of them all, which makes the 'merging' mechanic a bit repetitive after every mission. It can also get a bit frustrating when you get the 'you have max number of monsters' all the time.
I like how TsumTsums handles it where at the end of a level, if you roll an unlock for a character you already own (even if you are not using that character at that time), you get bonus XP for them instead. You lose that extra control over who fuses with who, but it keeps the game running quickly.
As for getting tired of the combat mechanics, this is my big concern at the moment. I'm a bit worried that it might start to get repetitive after a while but I've not come up with anything too smart to solve that yet. Probably kind of silly of me to only be thinking of this now after already implementing these systems
Be sure to post a version of your game when its ready I'll play it!
There's a link in my signature if you're interested, it goes to the developer journal that has download links at the top