For my money:
Aleksandr Vlasov - could possible step up and become a GT winner contender, but at the moment he hasn't been showing enough consistency.
Ben O'Connor - I don't think he will be a GT winner contender. To to that he would have to up his game on the ITT and become consistent at a high level in mountain stages.
Daniel Martínez - I don't think he will be a GT winner contender, whilst at INEOS. If he leaves INEOS than I think yes: he has top mountain skills, consistency and decent ITT.
Enric Mas - to me he will continue to be a podium type rider, thanks to his enormous consistency in mountain stages. However, we hardly see him exceling and his ITT is dreadful.
Gino Mäder - he has been a positive surprise this season, and only at 24 years old. I wouldn't be surprised to see him upping his game to a GT podium level, and who knows if from there he could be a GT winner contender.
Jack Haig - to me his 3rd place at the Vuelta is as good as he is going to get. I can see him podiuming a few more times, but at 28 and with 8 GTs in the pocket I can't see him becoming a GT winner.
João Almeida - together with JV is my top choice for a GT win from this list. He has the ITT, the consistency and he has shown this year to have the world class climbing skills - perhaps not to win the Stelvio with two minutes to everyone, but if he is able to climb with the best and then have ITTs only second to the likes of Ganna, he is a GT contender on any GT where Roglic isn't (I'm not counting with Pogacar as they will be team mates).
Jonas Vingegaard - his Tour was a positive surprise. He needs to do that more often throughout the season for me to rank him above the likes of Almeida, or on par with Carapaz and Bernal, but we certainly want to see more of that and only on his second WT season.
Miguel Ángel López - doesn't a chance. He is lacking the ITT skills, consistency and brains.
Remco Evenepoel - he can either become the next best thing after the Slovenians, better than them or just a Classics rider with no 3 week GT capacity. He has the talent to be the best, but a lot of work to do on his bike skills and head shape.
There are other names that could be added to this list, as others mentioned: Foss, Rodriguez, Ayuso and the Norwegians, just to name a few.