I think this sounds like a really cool idea, and am certainly interested - you can get a lot of interesting gameplay ideas out of looking at the interactions between different types of character across the 41st Millennium, and seeing how a devout member of the Order Famulus, a warped Navigator, and a Space Shark interact, work out their theological and cultural clashes, and address problems together would be really fun! Count me in, though I’m not sure as what …
My two thoughts, since you’ve requested them:
- As an old game, Dark Heresy, in my experience, has always played a bit hinky when run alongside the other three gamelines, especially when run as a low-level Ascension character - they’ve been picking up 100 and 200xp advances for years, and end up as monstrously effective, but fairly fragile Swiss army knives next to the other characters. Space Marines have their horrifying armour values and destructive potential to make things up, but Rogue Trader characters tend to pale a bit compared to their Darkly Heretical cousins, especially when they’re not given free reign to swan about throwing profit factor and macro cannon fusillades at things. It might require a bit of hacking in order to make work (Faith mechanics, etc), but I guess you could make some Rogue Trader-equivalent Sisters as a Regimental background in Only War.
- As you say, combat-heavy is an issue in forum games. My advice would be to take advantage of the fact that all three types of people are going to be masters in their own fields - Astartes are incredibly hard and unlikely to get chumped by a handful of Orks, Rogue Traders have limitless staff and arms men, and Ascension-level characters have significant political power in the Imperium. If I were running this, my temptation would be to have a central mission control for planning/negotiation, and, from there, have characters run missions and activities that were rules-light, and largely based on skill checks/broader stats. So, rather than playing each move of a Kill-Team’s assault on a hive of minor cultists, the player and GM resolve it in general terms, telling a story whose highlights are determined by dice rolls (they seem to be overwhelming you, command check! You’ve got a clear shot on the psyker who’s enslaving these people, roll an aimed shot from your bolster!). Then, when shit’s really about to go down - the Genestealer Patriarch with a Halo Device is opening a warp rift to free a trapped C’tan - you switch to full combat-heavy rules-heavy play. It might take a bit of getting used to, but especially with the higher power (personal and organisational) of the players, it might be easier to run.
My first thought would be how well do the systems mesh? If one systems has characters sugnificantly stronger than the others (the Marines most likely), is that going to leave other PCs playing second fiddle to a Marine in every scene?
Barring edge cases, my experience the overall power level (purely in terms of FIGHTAN, and overshadowing during combat) goes
DH Inquisitorial psyker -> DW Librarian -> Optimised Rapid Fire Ranged Weapon Astartes -> Optimised Rapid Fire Ranged Weapon Non-Astartes (Sister of Battle, Secutor, guardsman with tricked out auto cannon) -> Focused Human With Lightning Attack and a Horribly Good Weapon -> Focused Space Marine With Lightning Attack and a Horribly Good Weapon -> Techpriest or Guardsman Tank -> Sniper -> Combat Generalist -> Non-Combatant Academic -> Non-Combatant Diplomat.
The higher-up ones can trivialise encounters and overshadow people in combat by flinging high-psy-rating powers/dozens of full auto-shots downrange at the start of a fight. In practice it's not Astartes who necessarily overshadow everyone else - rather, as I suggested above, RT characters can have the toughest time of it in fact, hence the suggestion for a slightly more rules-lite skill-based approach for people who want it.