Is there any plans on making the undead faction playable? or are they going to be strictly like enemies?
This is actually quite an interesting question!
I can see it now, with their own dilapidated buildings, and human farms …
A bit dark perhaps? Rest assured somebody will mod it in, or perhaps you will create it!
As of now, I’d say not for the foreseeable future. There’s already the three kingdoms and the Dwarves as playable, with the Dwarves coming post-release, I would imagine then that if something like this was to be in the core game it wouldn’t be high on the list of features to get in there, and as such you would be waiting for a while!
But as I’ve said, It’s all achievable through modding
Extremely easy. As long as you keep the same model dimensions that humans use (so that the animation bones provided by Radiant still work) you can just swap skins.
If you want to add some depth, like spawning new workers from graveyards or fallen enemy corpses, necromancers and whatnot then you will need to mod it in properly.