First things first, awesome application here! Currently waiting for my card while teasing myself in the showcase section, aha!
Anyway I’m one of those people who like the little gimmicks and whatnot, so I thought having theme! unlocks, maybe little avatar unlocks could be cool. I first thought about more you spend the more unlocks, however that wouldn’t be good for the people who use the card often but little amounts. So the better idea would be how much you save, although that could be abused.
I would love to here your thoughts on what else you could unlock these with.