I'll admit that I never understood people's desire for a Battletech grand strategy sort of game. Sure, you've got big armies and such, and navies or whatever in some eras, but it just sounds so antithetical to what people generally appreciate about the franchise.
Yeah, seriously. The big draw is the BattleMechs themselves but you need to get down to the very small-end of tactics for their to be appreciable differences between them. The smaller, the better for that sort of thing. Once you're throwing battalions against battalions, who cares, since they're going to aggregates of combat power, modified by speed and survivability.
And even if that thorny problem were cracked, I have personally never played a 4X empire builder that had declining resources and tech scavenging rather than traditional R&D result in satisfying strategies. It is always some variation on turtling and fighting as little as possible while letting other factions burn themselves out by going to war and suffering (irreplaceable) losses.
I re-read this thread title and realised my first answer wasn't radically different, so here's an idea: engineering managment game, like those XYZ Simulator games. Lead a team of Mech Techs maintaining and repairing a unit's equipment, all while dealing with supply issues, parts issues, personnel management, dealing with "client-side" complaints (the MechWarriors unhappy that walking their Mech into close combat means they can't have it back in pristine condition for the next battle 3 days later, the Company quartermaster unable to procure the right weapon, so now you have the privilege of telling the merc commander that no his ER PPC is irretrievably broken for the foreseeable future, or if you're working for a House military, regimental HQ telling you that oops you're going to have to work in field workshop conditions for a month longer etc.)
That has a lot of potential to be thematically in-tune with the rest of BattleTech. Late 3rd SW, trying to keep a House unit or merc company mission capable, having to juggle priorities and keep plates spinning at the same time.