I'm hoping that they don't just limit the additions to the events that take place after the magical elevator ride and that they add cutscenes during the mission in London that reflect player choice. For example, if Grunt survived the suicide mission and the incident with the Rachni, we get to see him lead Aralakh company during the battle, saving Wrex from being overrun by Reaper forces. If Grunt is dead, Wrex goes down in a blaze of glory. If Wrex is dead, we see Grunt dying trying to save Wreav. If they're both dead, we see Primarch Victus die because of weak Krogan leadership on the battlefield. Moments like that wouldn't require any new gameplay and would allow Bioware to claim that they maintained the "artistic integrity" of their star child nonsense, but they'd give players the feeling that choices mattered.
However, there's still the problem of the relay network blowing up and leaving all your choices meaningless.

Honestly, I can't see a way for them to keep that in the ending and properly rewarding player choice at the same time, those two things appear mutually exclusive.