My biggest problem has been coalescing justification of identity. Death Guard lineage is easy to go through, but it's hard to make Nurgle Iron Warriors or Alpha legion because ther operation modi are disimular. For me in retrospect when Legions have been faded and the distinguishment has devolved from "you say they're Iron Warriors, but all those Plague marines don't make it different from the other," I lose interest in fleshing details because in the end, they still feel the same.
I've been through the Putrefactors so many times I'm sick of trying to even brainstorm. Quite honestly not having a specific favorite Legion, Chapter, or Warband hurts someone who bleeds explosive ideas and concepts. My order of the severed head for nurgled AL lasted a few hours before I said nm.
I understand this completely.
What I personally feel is that taking a non-Legion is best for those situations. I love the inspiration of a hardcore legion but when you want to flex some ideas it can feel... awkward.
But for your thought of Nurgle Iron Warriors (for example) i always used 'bitz' to represent and hopefully those bits weren't TOO focused on a mark or else I would be locked into that mark which sucks. For example, my Iron Warriors with heavy Iron Hands bionics became Plague Marines, or just MoN or just no mark at all.
I had terminators with a glowy mechanical piece on their backs. I used this tech to represent Tzeentch (increased save) but it wasn't too obnoxious looking and I could just call them 'unmarked' if I wanted to, or even use the 'tech' as an 'agitator' (MoK).
Those are ways I found around those issues, but going back I have indeed played hardcore Deathguard through a tournament season, and did well, and then sold them. But if I revisited that, I'd probably do a Nurgle-esque Chapter:
I can't remember their name but they are those guys with the fly on their shoulder pad. I'd probably do something like that to keep options open just enough to not have marks, or 'oddball' marks on some units.