Yeah, my plan is to basically have multiple branching upgrade paths, where you can either go pets or towers. In theory, maybe an army of pets could hold things off just as well.
I'll definitely be adding more mechanics to the upgrade selection once I add more upgrades and paths to choose from. It may be that even instead of a hard banish, certain choices will lock you out of others. I could go with a banish though, and of course reroll. Pets will probably be purchasable with coins just like tower guardians are, but will be much more expensive since they're permanent.
I plan to add dogs next and lots of other animal options to follow.