All great games. I have to play whatever you come up with!
If you haven't already, you should build the tutorial. I know it's a platformer and not what you are looking for in the end, but there really isn't much difference between the two. If you build that, then you will be ready to build a couple proof-of-concepts. And seriously, build the proof-of-concept first, not your full game.
In a sense a 3/4 view beat-em-up is actually a top-down game. It might be easier to think of it that way instead. It's a top-down, but your sprites are drawn like a platformer from the side. Even though it doesn't look like it. So after you build the tutorial, build a simple top down game by setting player and enemy gravity to zero and adding up and down movement bound to keys. You might also have to customize jumping since gravity will be zero, or turn gravity back on during jump?
Up and down movement will probably also have to consider Z as well as Y. This will keep the sprites in front of or behind each other when they need to be.
The next hard part will be working out your hit boxes for bad guy ass kickin'. If you are standing closer to the screen than a bad guy standing farther away, but your fist still goes through him, you don't want that to count as a collision. It won't make sense. I think I saw another post in this group that answered this question but with bullets. Take a look.
That's my rambling about it. Good luck. This sounds like fun, I will have to try something like it myself.
- Scott