Well snake's out because we already have Serperior, Horse is out because Ponyta, and Tiger is Entei. It'd be too similar. That leaves Goat or Rabbit. I'll go for Rabbit.
For next gen, yeah we're definitely not getting another snake but we did get Turtwig a couple generations after Squirtle so maybe later on. Similarly, I think they still can do a Horse since Ponyta was introduced a long time ago and it's not like old concepts haven't been revisited before. Especially if they can derive the starter from a different mythology. Also, Entei isn't a tiger but a Shisa like Arcanine.
I'd like to see the ox done (and knowing GF, it'll probably be based on the Buffalo King from Journey to the West), though tigers and rabbits are also concepts I'd love to see GF tackle.