A vampiric illithilich, obviously.

In general, it would be awesome to be able to become a lich through some difficult, obscure means and a lengthy wizard quest. But I suppose it would be better suited for a sequel/expansion with higher levels, so we don't have the "level 8 lich" dissonance. (CR 21, so would be a difficult encounter even for a party of level 20 practically-demigods.)

I'm going to agree with Sadurian: custom vampire spawn would be cool. I'm not a fan of Astarion, but the mechanics seem interesting (invitation, avoiding water). Also personal quest of killing the master for promotion to full vampire. Aaand I'd love to play a race that is not normally seen as vampires: picture vampire halfling, dwarf, dragonborn or half-orc.

I'd also like to be able to play as a wererat. Wererat + illithid tadpole = were-cranium-rat.

Or a werewolf tabaxi. Would confuse the hell out of everyone. (Werecat tabaxi might be even more confusing, though.)

I like the idea with the Hand and Eye of Vecna. I'd love it if the game gave us some cursed alignment-related items and actually restricted dialogue options if we "use" Vecna's remains or items like Helmet of Opposite Alignment. (Though I suppose that's not likely given WotC's current alignment policy...)