@Bessemer
Yeah, that's especially why I was looking for input/looking for people to refine & check my work.
The challenges in trimming a list down to fit a certain theme are:
1) Do I even have an appropriate/correct theme/play style in mind for this fleet?
2) Do my restrictions and selections actively support that play style I have in mind appropriately?
and
3) Am I being too heavy-handed in my restrictions, forcing certain fleet composition decisions rather than encouraging them?
It's not easy to answer those questions on the scale of an entire fleet list, and especially not if its a project you're invested in.
In the case of the Khornate fleet, I was trying to skew the list options toward vessels with medium to close weapons range bands or above average mobility not simply because certain ship types seemed to fit Khornate archetypes, but because I hoped that the remaining ships as group, with the mark and upgrade available in the list, would encourage a certain baseline play style.
As an example: an Inferno-class cruiser is basically an alternative armament Murder ( I think of it as "Plagueclaw pattern") with the 2 60cm prow lances swapped for Str 6 60cm L/F/R batteries. It's a minor change with immense consequences. There is no need for an Inferno to close head-on to the enemy fleet to maximize its firepower, and like a Carnage it can run abeam to the enemy fleet at about 45cm out, pumping salvos into the enemy fleet and moving to encircle them.
There is nothing wrong with that tactic at all, in fact it is largely how I used my Carnages and Devastations and Acherons and Styxes for years, but that didn't feel terribly Khornate to me. So, by relegating Carnages and Infernos to reserve options (off the list doesn't mean out of the fleet necessarily!), the Khornate line cruiser options shift to the Murder and Slaughter. You could use Murders and Devastations and Unbelievers to circle at a distance... But you would be wasting your prow armament. Alternatively, without the Carnage as a go-to line cruiser, I was hoping a player would be encouraged to instead play to the fleet's massed prow lances advantage (and the moment you line up across from an Imperial player with 8+ locked-on prow/dorsal lances aimed at their command vessel, they begin to forget about the "advantage" of nova cannons for a bit and push to get abeam of you quickly!), which would by necessity leave them eventually mingling their ships with their opponents somewhere near the middle of the table, at which point doubled boarding values and the ability to double your attack craft for a turn can crush softened targets.
I'm still not sure about my grand cruiser & battleship selection options, in fact I'm having a hard time justifying it to myself at the moment because it seems to play to 40k Khornate stereotypes and not necessarily Khornate fleet tactics. That said, it's tempting to hang back with a Despoiler, and not at all with a Relictor (see that point about whether or not I'm being too heavy-handed instead of encouraging).
My hope was to encourage/guide players toward Khornate tactics by limiting ships to those that would support those tactics rather than building a fleet of "archetypically Khornate" ships. I realize it is a bit of a fine distinction.
Slaanesh and Nurgle will comparatively be difficult to theme. Everything uses the same ships, but "how" is the hard part.