It was built for this jam, so the "one-button" idea limits some things. "Very plain," how? What would you like to see? I think I'd like to keep developing this game after the jam, so specific details help.
Yeah exceeding it beyond singular input would really bring out its potential. If I would consider a regular game, it is not plain at all. But for this specific genre, I'd say much more complexities and rules being added would spice it up. I'm presonally not a big fan of such game (nothing personal obviously, I do understand the appeal and your game executed it pretty well). But it felt light, lesser packed like usual such games are of this genre.
I'd suggest some "stealing" mechanics on specific combos, like taking 500 from the rival, if they go bankrupt they lose, and many other stuff. Rooting for your future development of this game, best of luck !