Hmm, I am quite sure that the citadel DLC will introduce the MP weapons to SP (the ones that are not there already), I am speaking of:
-) Lancer
-) Suppressor
and (hopefully)
-) The Collector weapons (Collector assault rifle, smg and sniper rifle)
and maybe, with a good buff, the Geth Spitfire
New weapon mods...
-)Sniper rifle high velocity barrel (I am pretty sure about that one)
I am REALLY HOPING FOR
-) Pistol and SMG power amplifier;)
About the DLC itself...
Shepard's private room will be in one of the apartment rooms on "Citadel commons" and I like the idea of it being possible of adding your own touch to it
Then, an actual casino...
Looks like we will get a new source of infinite credits (well, the gamble in the pit fight in ME2 was also an infinite source of credits, but good luck waiting)
And a shooting simulation like back then at pinnacle station in ME1 yay
The shooting range we already have is kind of lame after all
All in all... The citadel DLC is WORTH THE MONEY
The Omega DLC wasn't bad at all, it was well made and epic, but wtf are the parts where the music is simply missing? Yes, at some parts you really EXPECT there to be dramatic, victorius, tragic... music, but all you get is the lame background sound and zero music

That made Nyreen's death kind of disappointing, just look at the scene where Grunt dies/barely survives when fighting some Rachni while Shepard runs for it- that scene had a great soundtrack