I’m not really familiar with RE9, but a some surface level research suggests that (mechanically speaking) mutation is just “the abilities of bad guys change mid-fight”.

That seems like it is easy enough to bolt onto more-or-less any game.

You just need a variant stat-block for the enemy and something to trigger it.

D&D 5.5 has a bunch of monsters with abilities which change when they are on half hit points but you can use other triggers such as “is the target is a spell” or “the GM decides the players are having too easy a time and spends a Fear point”.

The trick is to come up with mutations and associated mechanics which fit the narrative.

Not only system, but also how you are using the system.

Some games are built around resource management. Travel rules solidify the way in which travel saps resources (like supplies and time) and force choices (every day we delay people die but if we press on through the night we will get there tired and hungry or might run into something dangerous in the dark) while also (with random encounters) showing that the world exists beyond the confines of the plot.

Some games are far more “epic hero”, the protagonists are the centre of the world and what they aren’t involved with doesn’t really matter.

It’s a good idea for a GM to know what type of game they are running and for a system designer to know what they are trying to design.

If travel rules suck then there’s a good chance that the GM picked a system that isn’t a good fit for the game they want to run or the system designer shoved travel rules in because they thought the game should have them just because it is an RPG.

Obviously, there aren’t absolutes and some games work perfectly fine if you jettison their resource management aspects as a house rule. I think it is useful to try to understand why rules exist before changing them though.

Amber Diceless which compares stats with GM fiet based on the situation.

Everway which also compares stats but if things are close has the GM interpret a picture card.

DramaSystem which is designed for PvP play and trades tokens for conceding a scene (and multiple tokens can be spent to force another player to conced).

Fiasco which does use dice, but not (as random number generators) to determine outcomes. A player, on their turn, can choose to establish a scene or hand that responsibility off to the rest of the group. Whoever doesn’t do that picks if the outcome of the scene is good or bad for that player’s character (subject to the availability of good or bad tokens in the pool). Second editor ditches the dice entirely and adds cards instead, I haven’t played that version.

For the Queen isn’t a traditional RPG. It provides a card deck that asks questions about characters. Figuring out the answers lets everyone learn about all the characters (including their own).