To start off- This isn't a necessary change or addition at all, and I understand it'd take a lot of work, but who knows? Definitive edition might come with surprises.
Some Origin Characters have very customized romances, and I was thinking about how some romances play out for companions- I think it'd be cool if when you play as Astarion and ascend, you got a dialogue option to propose to turn your LI into a spawn- basically in the same dialogue tree where you can ask for kisses and stuff, and in the vein of discussing the tadpole.
I imagine most of them wouldn't accept- (from the clues we have, it seems Minthara would) but I do believe /some/ might cave with a persuasion or deception check (telling them you will turn them into a vampire lord). Karlach for her engine (even though if she's a companion and you're playing another character, she doesn't approve of you becoming a spawn, I think the engine+being the one in love herself might make her doubt) and I think it could arguably be an option for Shadowheart, and perhaps Gale if he's seeking immortality. I don't think others would accept, and from that the player should be given the option to end the relationship because of it or not care about that.
In Karlach's case of course, the attempted transformation would lead to failure, but since we're playing as Astarion we should get the option to decide how he feels about that.
I think the cool part here would be their reaction if you do turn them- waking up as a vampire and stuff, and how they feel about that. Hopefully it'd come with at least an eye color change, too, and impact the Elfsong Tavern scene/the epilogue.
(I'd argue that, in this vein, Ascended Gale could also propose ascending to their LI at the epilogue party, for example!)