I suspect Liara will buy a ticket to the big blue farm in the sky. In ME2 they really played up the intimacy between her and Shep in LotSB, far more than with Ashley on Horizon - even if you'd shot down Liara to romance Ash in part 1. In the ME3 demo both appear but you've got Liara as a squadmate for half the demo while Ashley doesn't even rate a handshake. I suspect they've locked in this closeness between them regardless of the PC's relationship choices to set up some big dramatic payoff in a cutscene down the road (i.e. heroic sacrifice or surprise death. Aside from Anderson (who might have a bit of martyring to do himself), every other character from earlier could be dead already so there aren't many other options for a late-game "now it's personal" event.