Novice researching RPG rulesets

First, please bear with some initial context:

I am creating a MUD engine from the ground up. I am nearing the point in development, where I will have to carve out a player/combat system, however I admittedly have zero experience playing TTRPGs. The only understanding I have is of being a player of existing MUDs and RPG computer games.

My understanding is the majority of MUDs are based upon D&D rulesets, occasionally modified to varying degrees. I would personally like a bit of an edge, and model my system after something else.

The engine is geared towards worlds that:

• Encourage solo exploration and survival.
• Have distant sci-fi, cyberpunk, and dystopian themes.
• In addition to equipment, implant, and class bonuses, will incorporate gunplay.

It would be ideal to base off a ruleset that isn't [[[extremely]]] complex, but also allows for customization and flexibility.

Any suggestions are appreciated, and if there are any further questions, please ask.

Thanks!

EDIT: If anyone is curious about or wants to follow development, here is the github repo.