Given the reactions we're supposed to have it seems more and more likely it's going to be Giles, Xander, Angel or Spike 🙁
There's a couple of problems with that though. Would Joss step on IDW's toes by including Angel or Spike? How was Giles and Xander in two places at the one time? Did Xander really shoot a missile *at himself* in 'Time of Your Life' and would any of these characters really opt for *genocide* for the greater good?
The problem with making any of these characters Twilight is that he's running the risk of doing things that it's hard to come back from. He's working with Amy and Warren who sadistically tortured Willow and tried to lobotomise her, he sent a missile into the Slayer HQ which killed a number of slayers and he's wanting to wipe out the entire slayer population, killing thousands of young women. Could fans ever love these characters again?
They all fit and at the same time they all don't. Xander and Giles would easily fit the "closest, the most unexpected" line. Giles and Angel are big picture thinkers so they could be doing this, Angel fit's the height and shape of Twilight. Both Spike and Angel already are in the superpower department so less fiddling has to be done with that, and we don't need an explanation from either how they can in two places at the same time. Giles was with Faith when Twilight was with Lt Molter in "˜No Future For You' and Xander was in the castle when Twilight was hovering above Buffy and her slayers on their mission at the start of "˜The Long Way Home.' The "I hate to see you cry" line could easily fit with Angel, Spike, Xander or Giles and the "itchy neck" humour could easily fit with Angel, Spike, Xander or Giles' sense of humour as well.
Honestly if I had to pick the most likely out of those four I'd say either Giles or Spike. Giles is a big picture kinda guy, Xander isn't and Angel's the main character of the whole other series. Would Joss really risk loosing that and keeping him over on Btvs for such a long time? Spike isn't the star of the show so he could probably be used between both series more easily.
My best bet out of those four.. Giles 🙁