I think the important point of this and similar "what's next/endgame" threads is that Factorio has such immense potential that those of us who have been bitten by the bug/"get" the game (took me 3 hard bounces off of it before I started down the 500 hour rabbit hole (of joy)) simply hate to see what ...

Mechs with decent RTS controls would definitely be a great addition. Using our complicated factories to effect RTS explodies is the ultimate endgame.

Robot Army mod is neat but the controls are pretty limited and unreliable.