It’s either going to be vulpera for horde and mechagnomes for alliance in 8.3.5 or they will both be neutral and available to both factions.
So far every allied race that has been released has always been the same racial type.
Void elves/night elves=Elf
Mag’har/orc=orc
Dark Iron/Dwarf=Dwarf
Lightforged/Draenei=Draenei
Highmountain/Tauren=Tauren
Nightborn/Blood=elf
Kul’tiran/Human=Human
Zandalari/Troll=Troll
There has never been an exception and the important thing to realize is that Vulpera aren’t goblins. And the highly speculated mechagnomes are following the same route. Not only this. All allied races have shared some sort of intrinsic similarity with existing race. Gilgoblins fit a lot of the criteria that falls into being a potential AR for goblins.
Sounds weird right? 2 neutral allied races?!?!
Blizzard isn’t fond of the idea of neutral races. They have gone on record saying that they didn’t like how things turned out with Pandaren as a neutral race.