Re: Movement, one thing that became very quickly apparent in 3E with universal movement distances is how much more dynamic the game became. Even with the return of movement values in 8E, I fully expect many units to retain a movement value of around 6", simply because with the old RT/2E typical 4" movement, it takes forever for troops to move on a 6' x 4' board when a game meaningfully lasts about 4-5 turns.
Re: Squats, those are dead and gone. GW confessed quietly on several occasions that they tried to come up with cool ideas for them, and in the end simply felt that they couldn't come up with anything compelling. Therefore, after a long time on ice, they were finally written out of the background as having been wholly consumed by the Tyranids. Of course, GS Cults were recently resurrected as a faction with models, but then they had always remained part of the current background - unlike the Squats.
Re: Space Skaven, I don't think 40k needs more swarm/horde armies really. Orks and Tyranids in particular have a hard enough time already, and I'm struggling to seeing what Space Skaven could bring that's unique besides being man-rats... Most of their schtick is already covered by Orks.