Fun little base for a game. Here are a few things I'd like to see (bit late but hey, thought that counts, right? :p)
I would very much like to see additional powerups that can appear randomly in the shop, something along the lines of returning damage to enemies, or everythird attach leeches life, etc This would give the gameplay more depth in terms of choices to make on the fly. Something akin to the various items in Vampire Survivors?
I find the idea of stackable buffs really cool; definitely something to work off of. Maybe have a look at Heavy Rain on how you could stack various levels of weapons or items to get cooler effects.
In true roguelike fashion, it'd be interesting to see what you could carry over, some sort of small powerup or money, to make the subsequent runs easier, so maybe after every 3 or so levels, give each character +1 towards a main stat to mesh well with the first idea, or carry over 50% of the total gold held at the time, to make no upgrades during leveling a possibility.