Aside from that center of government being filled with dead bodies and hopefully dormant Reaper tech? Why would they go back might be a better question. The Citadel is a sign of the old era and the Reaper cycle, not the new. It was the hub of the trilogy, but is not the trilogy. This new character is not Shepard, Citadel Council Spectre, but apparently isn't even the same sort of space marine type at all. Why should the Citadel remain the hub, when this is not the same story or the same characters?
They can introduce new races, they can introduce new locations, but none of that really fits with the immense distance implied by the devs. We've zipped from one end of the galaxy to another with the Mass Relays. If its on the network, where's the distance? It just means a fraction of a second in a relay. That's not new or surprising. If it's not on the relay network and it takes years to get there, then you can forget about revisiting the Citadel or any old locations any time soon. It may as well be a different galaxy.
It's quite plausible. This is science fiction. Say, perhaps, the ark project leaders worked out a way to use the Citadel relay to send themselves across the great intergalactic void and launched just as Reapers were overtaking the station. Or the Asari, in their study of Prothean technology, developed stasis pods with a stable power source. All plausible.
Why do you keep focussing on the "Immense distance"? They said it ONCE. The thing they've really been emphasising is scale of the world. Casey even said "one end of the Galaxy to the other" not "across dark space and into another Galaxy".
Also, I don't know if you remember but EC ending (mine at least) had Relays and Citadel repaired with people using them. The Citadel was above Earth, they don't need Relays to get to it anyway. They rebuilt it and continued to use it. They rebuilt the Relays and can move around the Galaxy again. We already KNOW they went back.
"Ark project leaders". Can we stop with this? It is literally the stupidest idea - it's not happening, it's not plausible. The Council didn't know the Repaers were coming, how did they have time to invest in a massive ark project, keep it secret, and then just send everyone away into darkspace to die just as the Reapers came? I seriously doubt that in a couple of months some Asari scientists did what the Protheans did but 100x better, they weren't planning a slingshot across Galaxies. How would they even have time to work out how the Citadel works and then get thousands of people fired into dark space in 5 minutes? And what would they do once in dark space? Die, that's what. Cryo-stasis is plausible but then is ME4 just going to be a tonne of frozen Asari drifting through dark space?
Seriously, man. There is no ark project. It's not set in another Galaxy. IT'S NOT PLAUSIBLE. And even if it was IT'S A TERRIBLE IDEA. I seriously don't get where you're getting all this crap from. There's been nothing to indicate any of it. I am 99.9% sure ME4 is set in the Milky Way with all the locations we know AND NEW ONES. They're building upon what they've already got, not tearing it down after 7 years and starting over. How stupid do you think they are?
edit: going to sleep now, hope some people come to their senses and stop coming up with outlandish ideas that just aren't going to happen. This isn't even speculation, it's just pure fan-fiction.