This is actually an excellent plot idea, with Gaspard, you know. Lavellan agrees to marry Gaspard for political reasons. Then she conspires to kill him, thereby becoming Empress of Orlais upon his death. But Gaspard is wily so it wouldn't be easy...
Andraste did have children. It's just that at first because of her apparently frail/weak form, she wasn't believed capable of bearing them and surviving. Don't remember how many of them were actually hers.
I think this idea's been done before with Dorian/Lady Inquisitor. Considering political marriages for appearances are one thing he stands against, though, it just kind of doesn't fit him? You'd need a super good reason for him to do that, rather than refuse on the grounds that he's not into women and he's not going to pretend to be someone or something he isn't for the sake of politics.
Well, if Clan Lavellan wants an heir, they're going to probably want that heir to be elven. I can't think of any other high ranking elves, unless it's maybe someone from the College of Enchanters (pretty sure you don't want the Circle of Magi involved, especially if you are a mage).
On the other hand...
Dear Abelas,
I hope this missive finds you well.
It seems I have recently come into possession of landholdings and a noble title in the city of Kirkwall. Seeing as you are a leader among your people, and I am a leader among mine, it stands to reason that there would be much to gain politically and socially from a contractual agreement. Would it be too crass of me to raise the topic of a potential marriage between us, uniting my people with yours?
Your standing and mine are more or less equal at this juncture. Since the Inquisition has disbanded, my forces have become much smaller. Yours were never particularly massive. Should we unite in marriage, we would both gain in martial strength as well as political. Elves would stand to benefit--alas, perhaps not in the way that you would prefer, but a place would be made for your people without the usual scorn and dismissal that my people suffer, while my people could finally begin to live away from the shadow of that scorn.
This is a chance for both of us, and I do hope you will consider it with sincerity.
Mythal enansal,
Former Inquisitor Lavellan
(Abelas would probably burn that letter...)
Also, regarding a political marriage with Varric:
It's possible Varric might agree to it if he were feeling similar pressure to produce an heir. But, if Lavellan is the Keeper's First, you'd probably be expected to have little elven babies--which you won't get with a dwarf, although if the theories and hints about Sandal and his origin are true, you might end up with a dwarven mage. I'm pretty sure dwarf/anything hybrids are pretty rare in Thedas. Not very fertile if not with other dwarves, maybe? Whereas according to the Orlesians at least, elves breed very quickly within their own species.
Much as Varric will always prefer and love Bianca, though, and since he says he's only into dwarves, I just can't see him even trying to produce an heir unless it's a Cadash he's trying with. I guess he could claim he tried and it just didn't happen. He does enjoy spinning tales. It's pretty sad he's always going to be hung up on Bianca when he can't really truly be with her... but I suppose the same may end up being said of Lavellan in the end, too.