Perhaps the engineers are going to take David's new alien and further improve on it in the third and last movie. I mean they should be really pissed after what David has done, and in their own planet, so double shame on them right
The goal then after improving David's proto-alien on the classic one should be of course to use it on us as retaliation.
The idea behind this is, first based on that pic we some months ago of two engineer on pressure suits examining a classic biomechanical alien contained in an observation tube, and second on Scott's recent line "AI shouldn't be that creative".
So all those who don't like the ide of David creating the classic alien should still breathe a sign of relief. Even then, David created the prototype