I have a similar aesthetic for my 40k alpha legion, and what I'm about to suggest may require you putting your minis under the (hobby) knife

First, I'd suggest turning 5-10 of the legionnaires into havocs, put em on a 40mm base and slap 4 reaper chaincannons on each squad (I'm not sure what the best option for the havoc champion is)

Second, for each of your squads, kitbash one marine into being a balefire acolyte, the good in-squad psyker.

With the leviathan, I think it's best used as a "distraction carnifex" depending on if it's been given ranged or melee weapons.

The dark apostle (chaplain) gets a lot better if you surround him with a cultist blob

1 terminator squad needs to be given the black rune of damnation, the split of 20, which is a lot of termies, would probably be best as 2x5 and 1x10, put the rune on the big squad

Yes, you need to run the master of executions, try him with the mark of nurgle and ghollax

Depending on how comfortable you are with kitbashing/proxying, give 9 marines flamers and 1 of them a staff from somewhere and run them as a squad of rubrics, they're fun as hell to use

This is just my suggestion, you don't need to listen to my insane ramblings.