I would hope that FFG would make Imperial Assault and Legion compatible, this would be a perfect opportunity for them to use the print on demand product line to allow us to buy stat cards for the Imperial Assault figures we have to make more varied and interesting armies at the outset of the game. It also allows those of us who have spent money on the other games to get more use out of them, and could spur sales of IA figures now that are un-purchased. If the two games were compatible I would have no thoughts about buying into this game as I would start off with a good sized army with the core and my current collection of IA figures.
If the two lines aren't compatible this is another product for my limited funds. I dropped the LCG long ago, am not into Destiny and I know many players at my FLGS that have dropped X-wing, or Armada, or Imperial Assault, or Destiny to focus on fewer games. If FFG made it so I could get more use out of the games I own I would buy more. As it stands if they aren't compatible I don't see myself getting this game as it is just another money sink.