2) X rays, Brutalities, and Fatalities wouldnt work!
They could make this work if they altered fatalities, brutalitles and xrays done on the alien for EVERY single character in MKX.. but that would take a lot of time and additional content... its not the same as how they altered fatalities done on Kratos in MK9...that was much easier. There werent so many obstacles to overcome...
I don't agree with the acid blood not working, because Reptile has a similar type of acid saliva / stomach acids that he spits up / vomits, yet NRS can make him work in MK9 and MKX.
This is the only part of your comment that I agree with.
Sure, I think that NRS could make it work if they altered every X-Ray and Brutality in MKX to fit the Alien's model, but I think it's quite a bit of work for them to do.
It's the same reason why Motaro probably wasn't in MK9.
The main reason why NRS probably didn't put Motaro in MK9 probably had less to do with hind legs and more to do with NRS not being able to figure out how to do X-Rays on him and how certain moves would work on him.
If NRS CAN figure out how to do this for Alien, then sure, I don't see why he can't be on the main roster.
Alien vs Predator in MKX would be huge, and even as someone who usually doesn't care for guests, I'd have to be blind not to see the business and cash potential for NRS of a lot of people wanting to see that.
As it is, we also have Commando Johnny (aka Arnold Schwarzenegger-lite, since Arnold didn't lend his likeness) and Carl Weathers Jax, which are great tributes to Predator.