Dawn of War Codex

in the form of new abilities and units ( through upgrades);

Are they made because the army has an roster hole or is stuff also implemented for refreshing the gameplay?

( Always considering there is some actual capacity to produce it)

Leaving aside the ability to produce it, meh. It's case-by-case I would guess.

Still, the question of production is a pretty big one

How difficult would it be to, say, combine existing stuff ?