Vue seems the obvious contender but I doubt completely replacing React with Vue this far into development would only cause a delay of a few weeks. Preact or Inferno seem far more likely, considering they won't require massive rewrites.

"A very large software company that use Inferno in production already had their legal team look into Inferno. It didn't infringe on any patents."

— Dominic Gannaway, creator of infernojs https://news.ycombinator.com/item?id=14783208

...yet.