I agree about the return of ability to rescue, no avatar (or one with far less focus), return to non-marriage support style of past games (if there's no 2nd-gen, anyway), and definitely more objectives. The "survive x amount of turns" type ones in particular would make me happy to see again. Weakening pair-up seems like a must, honestly. And money was just WAY too easy to obtain in Awakening, I never felt like I had to conserve at all.
More expressive character portraits than Awakening is my hope as far as cosmetic changes go, like if they could have a more fluid shift from one emotion to the other. And more face variety in the character designs, since Awakening had a lot of same-y faces on a lot of characters, or maybe it was just more noticeable there than in past games, idk.