Fighter as a class is only as cliché as the story behind it from an RP perspective imo. An assassin who has only relied on his mental acuity (precision and brain blades) and stealth to reach their goals deciding that they need to more formally learn how to be an effective combatant doesn't read as cliché to me, it seems very pragmatic. Or having escaped near death time and time again in combat, an assassin learning by trial and error how to improve their odds of surviving direct combat. Or literally extracting the knowledge via their soulknife abilities from every fighter they slay, adding experiences into their repertoire. Loads of potential there for a fun Rogue/Fighter build.
I have three questions that I ask every time I theorycraft an MC build (and some example prompts):
1. Why do you want to MC?
Is it to make a more powerful character/to diversify your skills/to fill a niche in the party/for narrative completeness/etc?
**2. Why is your character developing these new skills? **
Does your character have a latent predisposition to this change? Did they get challenged in a way that necessitated change? Did a party member/mentor influence them to try out a new skillset?
3. Do you want the change to alter your concept for the character (both in combat and RP), and how does that fit with the character you play?
Answering these questions helps me a lot in deciding how to explain the MC decision, flesh out the character concept, and work it in to the game as written. If you just want to mix it up or beef up your character, that is fine too, but I think it is more satisfying when you can bridge the RP and the mechanics.