How about general upgrades? Eg unit speed, regen time upgraded. Definitely need better heroes, after a few battles you have to keep him back and can't use him in battle, he/she dies right away.
But great game, although difficult...
Can you make the slot machine spin faster? It's really glitchy and often spins, stops and starts spinning again. Sometimes the last wheel (or whatever you call it) keeps spinning for ages and it gets frustrating when you're trying to get the Octo jackpot quest.