Hmm...have them not suck as human beings.
I mean really, NOBODY stays around to support Shepard, or at least contacts Shep prior to the Reaper invasion? Okay...they were busy...
Liara's on Mars, but doesn't bother calling...hmmm...too busy being uncharacteristically like a gangster I guess...
Jacob's busy cheating on FemShep b/c he hasn't seen her in 6 months...sounds logical. The man's a boring tool...
Set up Kai Leng and James Vega in Mass Effect 2, both as honorable and capable badasses. Otherwise, they aren't worthy additions. You can't compare these newbies to an established canon that's had over 5 years to live in the fans' minds.
Don't have Cortez and Traynor be the token LGBT throw-ins who have the romantic development of animals in heat (outside of romance, they are fine as throwaways).
Kal Reegar...just have him in ME3...that is it...just have him in it. Like Emily Wong...you don't kill interesting character pieces in freaking e-mails and meta-game tweets. At least to a video news feed to add heart, give their characters some scene to chew.
Javik...make him less of a jerk. I dunno how, but maybe have a loyalty mission tied to taking down the Reapers. Just something besides this genocidal bigot whom I'd eject out the airlock at first opportunity.
All this makes me sound bitter at BW. I'm not, as I'd hardly be this amped up for material that didn't absolutely deserve it. It's just fanboyism. Some people hate Aeris...not because she's a bad character, but because they prefer that Cloud end up with Tifa.
I dislike James Vega or Jacob Taylor...not because they're bad people, but because they'll never be as cool as Garrus and any of the interesting/meaty characters. Thus, seeing them alongside the more dynamic characters is an eyesore.
From a gameplay standpoint, Vega was an amazing addition. Did I ever use him? No. Why should I care about him? What makes him worthy of standing alongside titans that have proven themselves in either previous game?
Moreover, why should he be given a spot when others weren't? I'd much rather any of the suicide squad over Vega. It's like the NBA Dream Team debates over players who should have made the cut, but never did.
Make the staler characters more interesting, make them dynamic, give them grit and drama to define them, make them capable but show them as flawed. Most importantly, make them sympathetic by striking the proper balance of all these factors.
Hell, Mordin had more of an emotional effect in ME3 with 2 scenes than half the rest of the cast put together. Why? because of all the hard work that went into establishing him in ME2.
Why are Garrus/Tali/Wrex so awesome? Because of all the character development in that "boring" buildup hour or so on the Citadel in ME1.
Conversely, everything that Kaidan/Ashley did in ME1 was completely undermined by what happened in ME2. Their behavior, and lack of catharsis, in ME3 pretty much left me with nothing but hatred for them (Ash more so than Kaidan).
If I ever play the trilogy again, that ho is burning on Virmire, even with male Shepards.