even though I was disappointed with a lot of things in ME3 the characters were definitely not one of them
they are probably some of my favourite characters in any game ever hell probably even in all of fiction
thats why I bought the citadel dlc two years ago even though I was pissed by the ending
so yes they have very big shoes to fill
Personally, I didn't buy Citadel. They never fixed the endings, and I will stand my ground. The characters were done fairly well by BioWare - but characters is not where BioWare is weak. I hear they shined in Citadel, which brings us to my next point.
BioWare's weakness is plot in the Mass Effect series.:
ME1 - the Reapers are coming.
ME2 - the Reapers are still coming. Some day.
ME3 - BOOM, I told you so!
Along the plot lines lets just look at ME1. We discover that Reapers exist, and they wiped out the Prothean Empire and countless others before them. We race against time to prevent a Reaper invasion, which would be doom for us all. One Reaper named Nazara, or Sovereign, beat the crap out of the Citadel fleets and only went down due to a glitch it had when a posessed minion died mid-control. Had Sovereign kept its kinetic barriers up through "Reaperized Saren's" death, it likely would have won against not only the Citadel fleet but the Arcturus fleet as well, then opened the Citadel Relay and ushered in a new cycle. As a major plot point goes it should be noted that Reapers are insanely overpowered, and we have no hope.
ME2's plot was about how insanely overpowered Reapers are moving in on us, and apparently decide that the best thing to do is use Collectors to harass humanity and build a new Reaper out of human goo. Although it was possibly the best game in the series the plot was hardly moved forward. We were still as a character trying to convince everyone that the Reapers exist.
ME3's plot sees Reapers rampaging left and right as we struggle in a primarily hopeless battle. We are saved in the end because of reasons that defy stupidity - no organic made it this far, and we changed the variables (of what we have no real idea). Apparently we made them more powerful, and the Reapers let us choose their fate and that of the rest of the galaxy.
My point? (TL;DR) Plot IS important. Any writer worth their salt can tell you that. BioWare should not forget that point, and in fact should work hard on making serious improvements.