I just find the Legion to be an interesting "villain" in that typically, the villain of games is this outrageously overpowered force that'd be difficult to stop. In actuality? The Legion are, in many ways, the underdogs. They don't utilize tech as much as their opponents, they shun medicine and they specialize in melee combat. Then what's the Legion's main strength? They adapt. The Legion's main strength is that they're the only ones to truly acknowledge and face the challenges of the post-apocalyptic wastes instead of clinging onto old tech for survival. Aside from, say, the Gun Runners, no one else has a long-term solution; even the Followers admit medication is becoming an issue to provide.
In that sense it's this interesting juxtaposition where the "villains" are arguably the most qualified to survive. They beat Rangers and Brotherhood Paladins simply because while the former two rely on advantages to survive, the Legion relies on loads of experience fighting uphill battles to overcome any and all odds. For all the weaknesses they have within the tools they typically utilize, their strengths are that they're simply better, faster, stronger and wiser when it comes to combat.
At any rate, I'd hate to go back to having some typical villain that's truly powerful as all hell and we're the underdogs. It just seems so cliché to have good-guy underdogs these days, I think the Legion are a breath of fresh air and beautifully written.
So what say you all? Would you like to see the Legion return? Perhaps not in their entirety with Caesar and all as they are now in New Vegas, but in some way, shape or form see remnants of their culture and nation still alive and kicking for NV2, FO5 or what-have-you?