Nothing of what you've said makes Avallac'h a potential red rider...
1) In the books Eredin is the military leader of the Aen Elle commanding his red riders while Avallac'h is a spiritual leader. In the book Avallac'h is not part of the red riders.
2) While it is true that Avallac'h doesn't care that much about humans (he probably even hates them more than Eredin) he doesn't use the same methods like Eredin. Open violence is not his approach. He tries to reason with Ciri, Geralt and other people. Kidnapping humans as slaves in other worlds is definitely not his style. He stands "above" such daily life stuff.
3) Eredin and Avallac'h don't use the same method for their time/universe jumps. Eredin and the riders travel on the Great Helix while Avallac'h uses another method.
4) Ciri herself stated that Eredin and Avallac'h are two sides of the same medal but also embody two different principles: light and dark, day and night, good and bad. Avallac'h riding with the Wild Hunt wouldn't fit to that assessment.
5) Avallac'h seems to have no problem to show himself in his real appearance in the witcher world. He always appeared alone in the books, without the riders or Eredin. There is no reason why that should have changed in the games. Eredin's approach is violence while Avallac'h's approach is reason (which doesn't mean that he would use his own means of violence in the end...)
6) Avallac'h might be a powerful oracle and an elven mage but he doesn't seem to be really powerful in one on one combat. In the world of the Aen Elle he had to summon Eredin and his red riders when some Unicorns approached Avallac'h, Ciri and their companions.
7) Avallac'h is no subject of Eredin. They are on the same level. Fox and sparrow hawk. There is quite likely a reason why Sapkowski used these animals as a metaphor for them: the fox stands for cleverness and reason while the sparrw hawk (or hawk) stand for warriors and strength/aggression in mythology.
I think Avallac'h will and should play a role in TW3 but not as part of the Wild Hunt. Of course he is a supporter of the elven cause (just like Enid for example) and of course he detestes humans but I think he would try to accomplish his goals by his own means like he already did before. He might work together with Eredin in a general sense of the term since they both have quite similar goals (not necessarily exactly the same goal and definitely not necessarily by the same means) but he won't work FOR him. At least that's what I think and hope.
