I suppose this is as better place as any to post what I had in mind.
Basically, it's changing the ending, but not in the way you're thinking, WarpedWizard. You should recall if you sacrifice yourself you get that little extra clip after the credits, it's the same no matter who you marry, I thought of something last night while lying in bed, a way it could be different.
If your avatar didn't marry anyone, it'd be the clip we have now, if you're unit HAS married someone, then instead of Lissa being there, it would be your spouse, and would actually say something to you, For example...
[screen Fades to black]
Sully: Chrom, you know more then anything he can't just be left there right?
Chrom: What do you propose we do?
Sully: Ah man, the hell would I know? kick his ass up?
[sully comes into focus]
Sully: Ah..You're awake now...
Chrom: There are better places to nap than on the ground you know...here, give me your hand
[Player takes Chroms hands and is lifted up, camera focuses out a bit to show both Chrom and Sully]
Sully: Welcome back, the fighting is over now
of course if you married Lissa or Chrom the dialogue would be different too.