Ellenberger is a good fighter and similar to Hendricks. He is too tentative though and GSP would pick him apart with ease imo. Ellenberger would always be a step behind.
Hendricks on the other hand as he showed tonight is bat shit crazy and willing to throw it all out there to go for a knockout. He has devastating power in his hands and will run forward guns blazing, he had Condit in trouble a few times tonight and I consider Condit to be much more composed then GSP when under fire.
Try and think back to the last power puncher that really tried to bull rush GSP. I honestly can't think of one as both Koscheck and Alves fought much to tentatively. I can however think of multiple occurrences where GSP has looked a little dazy after taking a decent punch(tonight near end of the 3rd round for example).
If Hendricks can stuff GSP's takedowns in the first/second rounds then I see him posing a fair threat to GSP. If not then the only WW I think that might be able to beat him is MacDonald but I don't think that fight will happen. Maia might have a chance but I see his takedowns getting stuffed and picked apart standing.