Honestly, we should just be the Sith Empire again.
This is the largest Dark Side faction by far, and the one that closest resembles the flavor of the Sith Empire. When the original SE fragmented, the gameplan was to see which splinter groups stuck around and which did not - in other words, to organically determine the SE's true successor state. That was a long time ago, and I believe we have that answer now.
Seriously. As the Sith Empire, everybody wins. The Sith get "muh Dark Side" and the NFUs get "muh Empire".
To those who say it will somehow negate this faction's forward momentum, I couldn't disagree more. Could you imagine the splash it would make if this faction pulled together to do a quick, brutal and bombastic conquest of a planet, whether through a Dominion or whatever, and then topped it off with the galaxy-wide announcement: "guess what, motherkarkers? The Sith Empire is officially back, bitches." What a bombshell.
Why not just do it now? Really, if we recognize that making the faction's aesthetic a little more classically Empire-friendly will attract more NFU's, then what is the sense in delaying things? Want to see more Stormtroopers, Imperial Officers, Moffs, Agents, Generals, Admirals, etc.? Then make your faction reflect their presence.