since I’m absolutely untalented in game design I cant think of a 1:1 implementation of each idea, but first I’d do away with any “modifier” that’s color coded for a mere number tweaking.
the already existing enemy variants could fill that role themselves.
I mean what’s the tentacled poxwalkers more than an optical gimmick?
they die the same, are neglectable the same way etc.
now have them mutate 1 out of 5 in a horde on the spot with the x% chance of becoming something nasty.
no fatshark, not a chaos spawn but “green”, surprise me 
I’m a literal 40k noob so don’t know every nurgle employee, but from this vast stewing pot of his surely something suits the bill right?
them blue moebians for example… you had a radio operator some time ago, now let him do his frigging job and actually call in armored support.
which kind? again i dont care, be it aerial or these star wars clone at-hd walker whatevers.
or let a destructible piece of a nurgle tank plop through the marked wall and hail some shells at us until, like the wall-snot daemons there already are in other missions.
there’s tons of stuff to do without the lame rainbow mode and even without having to put “intelligence “ in the ai.
x triggers y if not prevented by the player…..
daemonhost rituals were a good start, now let them sacrifice a “captured” player for a more crotesque nurgle employee of the month
twins hard mode didn’t have a revive after death either