Djokodal are favorite, but next-gen have a good chance here.
Every GS tournament they had a better chance than the previous one, at least on HC, and this seems logical.
If you included Thiem's in next gen(I think he's a lost gen), i would say:
Nole 35-40%
Rafa 20-25%
Fed 5-10%
Other non Next Gen 5% (1992 and before)
Next Gen 25-30% (1993 and after)