I'd only be okay with Xenos if my opponent really worked hard to make an army in good faith, which would be very limited in scope and with lots of restrictions to mirror the design of 30K legions armies. For instance using the same force org with no formations or what not like mentioned above. I also think that all units need to be much bigger just like in Legion lists. For instance minimum squad sizes double what they are right now.
Example:
Defender Guardians are 20-40 in size.
Aspect Warriors are 10-20 in size.
If I was using big Legion Tactical squads opening up with Furry of the Legion into a mob of 50 orks or blasting them with Volkeit it'd be awesome, but it would be pointless to try and walk into a typical Tau gun line with power armor infantry...
I believe in book one they briefly talk about using these restrictions if you want to introduce Xenos into 30k. It could be done, but only very carefully...and in good faith!
