The rules don't strictly mention anything about the number of button inputs you can use in these games, but are we limited to the number of original GB inputs? eg: Directions, A, B, Start, Select. And what about mouse input?

Or does none of that really matter, just the visual style?

From what I've seen, it's just the resolution and colors that matter.

I kind of like the idea of limiting inputs.

Look at the problem from a different perspective: Part of Nintendo's early success was the limited number of controls in most games and their consistency. Sure, there was the odd game where you'd jump using "up" or run using "A", but ignore those.

If you can, try sticking to original gameplay controls, even considering it's not part of the rules. It might make the game more accessible and easier to pick up. Think about why "Flappy Bird" was such a huge success. I'm pretty sure part of that was due to the minimalistic control scheme.

Just try to avoid creating complicated combinations (not talking about something relatively simple as Castlevania's "up + B") you'd see in a bralwer such as Street Fighter without really needing that complexity.

I gonna try my game have standard GameBoy controls and can be playable with a gamepad, but i also gonna use Mouse so you can navigate and play the game with them.