Ship idea: Rebels TIE Fighter aka Sabine's TIE Fighter

I loved that. But, it was her and ezra who are already pilots on other ships. 

Cool idea. However for pilot it'd best be Rex or be pilotless. Since I can see CG making ahsoka fulcrum pilot the ship she does in the ahsoka show with Hujang. Since GLs don't usually pilot ships. That'd be where it'd make since there. I'd also like to see a scoundrel tie fighter from the mining guild. Maybe make that one pilotless. Also we could still use the inquisitor Tie from the rebels show be piloted by the healer inquisitor. Also it wouldn't hurt both sabines and the mining guild Tie being pilotless would be cool to see more of those versions of ships in the game aside from more of the droid ships for the Separatists.

Tie Defender came from the TIE Fighter game in the 90's. 

Yes, it did. However, Iden Versio pilots it in Galaxy of Heroes and prior to that had never been associated with piloting a Defender. What I mean is that GOH can take some creative liberties in assigning which characters could pilot certain ships despite previous media not associating them with that ship. However, Ahsoka and Rex have been shown to pilot the Rebels TIE Fighter in the X-Wing Miniatures game but not in the Rebels TV series.

"Current characters in the game who could pilot it are either Captain Rex or Ahsoka Tano (Fulcrum)."

While it does break lore (but since when does CG care about lore?), one could have Padawan Sabine be the pilot.

It wouldn't break lore as the X-Wing Miniatures game is considered canon. Even the Wookieepedia article for the ship acknowledges Rex and Ahsoka as having piloted it at some point.

Padawan Sabine could work, but at this stage since she's Spectre and not Pheonix it would be a stretch. Ultimately though, it'd be up to CG who pilots it. 

"Padawan Sabine could work, but at this stage since she's Spectre and not Pheonix it would be a stretch."
Jedi Luke isn't a Rebel.
Spectre Omega stopped being a clone.
Rose Tico was never the pilot/crew of the MG-100 StarFortress SF-17, it was her sister Paige.
Colonel Starck doesn't have a leader tag.
TIE Defender has bonuses for Tarkin's ship and not Thrawn.
Iden Versio is the TieD's pilot.
Admiral Piett is a trooper.
and just some examples.....

Like I said before, since when does CG care about lore?

 "Ultimately though, it'd be up to CG who pilots it."
100% agree.

Well then based on your argument here, either Rex or Ahsoka piloting the ship wouldn't break lore as it's already been established in canon in the X-Wing Miniatures games.