Liara: easiest fix is to replace with Shiala in ME1, recruiting an actual commando to fight as opposed to an archeologist.
Otherwise, I *did* like her LotSB persona, if only she could have kept that side rather than spend ME3 flip-flopping between tough/badass Liara and soft/shy Liara. I guess what I'd do is make that string of three paragon-interrupts after Vasir's death in LotSB determine how she is in ME3. 0-1 interrupts taken = hardened Liara. 2-3 taken = softened Liara.
Jacob: I'd have him rally some Cerberus factions under his banner to fight the Reapers, taking a Gen. Petrovsky role. Also, his ME2 mission should have been about something other than his dad being a creep. 'Not sure what right now, though.
Grunt: I'd have Shepard try to influence him rather than have Grunt blindly follow an aging krogan warlord's teachings.
Morinth: I'd have made her able to kill Samara during their confrontation scene, unless Shepard saves her with an interrupt or something of that nature. Important thing is to make it clear that Morinth is, in fact, the stronger one, so there is actually some reason to recruit her over Samara. After recruitment, Shepard can be apathetic to her killing ways, or try to steer her away from that and be committed to turning her life around. I'd also retcon the supposed pleasure-high from mating/killing.