Loving the way this is set up. Could use a bit more depth in the available followers, though not sure if that's already the plan. Something I'd strongly suggest is looking at some concepts in popular TCGs (not copy/paste from them or anything, but just certain concepts/features of those games that help them run more smoothly). For instance, giving certain abilities names (e.g. how MTG uses things like, "First Strike" or "Rampage" and other keywords). Systematizing abilities in this way helps make it more scalable.
It would also be nice to have a bit more flexibility in the randomness of the shop. Perhaps some ways to increase the number of items offered each time in the shop or even a limited, free reroll. It would help shift the balance between luck and strategy to a better place. Right now, it can be a bit frustrating to have a general build in mind and have the RNG completely shut me out of it. I understand that some stuff should definitely be random because the right rolls can make a totally OP build, but at least getting abilities along certain lines so I can at least strategize a theme that I'm interested in would be nice.
Also, in keeping with the idea of shifting from random to strategy, different variations of some of the followers would be awesome. Not necessarily a whole tier system (though this would allow for reusing art assets a bit to keep production cost down while expanding the game), but you could build some followers that are quite similar to existing ones but don't quite offer the same ability. For instance, you currently have a follower that gets +1/+0 on every attack - perhaps another follower could be made with slightly higher base stats but only gets +1/+0 each turn (for defensive build-up builds, but not quite as powerful with multi-attack builds).
Combining this added variation with more expansion on the shop would increase the chance of someone getting a build in the general ballpark of their intended theme without making OP builds easy to get every time.