I'd say you should make levels with the gimmicks that have question marks on them, especially the Turnaround on Assign and the new gimmicks that you added in V-1.35n, I think. Especially since you are no longer doing an LPIV Bonus Pack. Unalterable should especially have one since only one level (not counting Lemming Collider and Bomber Frenzy) uses the gimmick to its fullest potential.
Will there also be levels using combinations of gimmicks (in a similar vein to Party, only easier)?
For Zombies, will you also do Zombies on Death and Classic Zombies?
Also, despite the graphical differences, I would say go ahead with making levels with the other three LPIV styles (and Horror), since you are not making a bonus pack anymore and you're already using Wasteland.
EDIT: I'm still getting the same version as before (i.e. no new Mechanics levels, only the placeholder level)