Could also take a slightly left turn and make it Luke's X-Wing. Or his dead gunner from Hoth. Really, Luke spent very little time among other characters.
Thematically, it'll be hard to have different R2s in the game, because the current one is represented as a Rebel (Ep4) through Resistance (Ep 8), so I assume it's the continuum and they shouldn't introduce other variants inside there.
They could require Wampa, but the Hero Journeys were all about the team members, not the opponents, so I doubt it.
I'm fairly certain CLS, Capt Han, ROLO, Hermit Yoda will be required, and then I'm keeping an eye out for any Empire Strikes Back characters or ships with which Luke was closely involved. I expect the 5th toon to be released just early enough to be possible with 20k crystals, or a $40 pack like the Veterans were for RJT.