Most things like the risk style campaign map would be easly translatable to the space setting (it's basically like most 4X games). Provinces & settlements = Solar systems & planets. The main differences to the campaign map would be that there are no impassable terrain like forrests and mountains. It would just be the sea of space. But within that you could still have areas that affect attrition like asteroid fields and warp storms. Trade and Diplomacy would work pretty much the same. Ports would still be needed for trade with other solar systems.
- Factions like Craftworld Eldar and Tyranids would use Sea Horde mechanics like the Vampire Coast faction but with Craftworlds and Hive Fleets being the factions' upgradeable/moveable capital vessels.
- Eldar/Dark Eldar can enter Webway stance to fast travel and/or to see webway pathes invisible to other factions.
- The only mechanic that I'm not sure on is how the Dark City of Commorragh would work... The only way I see it working is if the Dark Eldar are made into a new type of semi horde faction. Instead of having moveable capital vessels like Craftworld Eldar, each Dark Eldar Cabal (subfaction) will have their own single access point (call it "X Cabal's Commorragh Gate") to their capital in Commorragh. The Commorragh Gate will be fixed to a static Webway Gate, but can be moved to any other active/discovered Webway Gate. To change gates they will need to perform a ritual which takes X number of turns (maybe depending on distance) and has a cool down period before the Gate (and all the Cabal's Armies within a catchment area) is moved to another selected Webway Gate. This will give other factions the window to attack/raise/close access to the cabal's Commorragh capital. If the Cabal has troops elsewhere when their Gate is closed they can perform a ritual to open a new one at any discovered Webway gate. If the Commorragh capital was raised it will need to be re-colonized a the Webway Gate where it was raised. Only Dark Eldar Cabals can capture/occupy another Cabals' Commorragh capital/Gate to have more than one Commorragh capital/Gate.
- Craftworld Eldar can also use (unoccupied) Webway Gates to fast travel their armies and Craftworld greater distances.
- Chaos can traverse the warp and Eye of Terror without penalties. Other races can warp travel like using the march stance. Necrons can fast travel without stats penalities (they don't use the warp).
- Non Chaos races suffers movement and attrittion penalties when entering the Eye of Terror, rifts and warp storms.
- Panets will have different climates and race suitability like in TW:WH
- Army/Fleet encounters in the sector of a planet will be fought on the ground map of the planet. Encounters that happen outside a planetary sector (ie in the Sea of Space) can be fought on Maps designed around the ship of the defender. Like how battles can be fought on Black Arks in TW:WH2 - That would mean no space naval battles (there's the Battlefleet Gothic: Armada games for that...) Siege battles will be siege battles on planetary forts maps.
- Space Hulks infected with Genestealers can be like the Mysterious Islands
- Corruption - Much like how it works now in WH:WH but given 40K flavour. Orks will now also spread Corruption/Infestations in the form of Ork Spore Infestations, Tyranid Genestealers Cults, Chaos Cults.
Basically you can compare the some of mechanics and play style of 40K factions to their FB (sort of) counterparts
Spacemarines = Empire
Sisters of Battle = Bretonnia...
Imperial Guards = Dwarfs (loosely because of focus on range/artillery and because the Squats are defunct in 40K)
High Elves = Craftworld Eldar
Dark Elves = Dark Eldar
Orks = Orcs
Tyranids = Skaven/Lizardmen
Necron = Tomb Kings
Chaos Spacemarines = Warriors of Chaos
Daemons = Daemons
No Vampires... closest thing are the Chaos Spacemarines Night Lords. What ever will CA do? Battles and Combat Units
The main problem is the squad based tactics used be Spacemarines and other factions like Eldar/Dark Eldar. Mass blocks of troops only suits Orks, Nids and Imp Guards. They could make the SM units squad sized like in DOW1&2 but make them uber tanky I guess. That would fit the lore somewhat.
Roughly speaking (strength wise not numbers); 1 SM squad = 1 platoon of Imp Guards = 1 big mob of Ork Boyz = 1 mob of Nobz etc.
The other problem would be the SM faction size if they're going to stick to lore. A SM Chapter is basically one SM subfaction with the chapter master as Legendary Lord. But a chapter only has about 1000 marines each, subdivided into x10 companies of 100 > x10 squads of marines > x10 marines in a squad. 1 SM company (1 Lord + 10 squads + 9 units of other units like scouts, dreadnaughts and other vehicles) would be = 1 army stack of 20 units in TW terms.
But then you have non-standard chapters like the Space Wolves who ignores the Codex Astartes and have more marines... Maybe non-standard chapters suffers diplomatic penalities with standard codex chapters?
Finding the right balance for the asymmetric unit sizes according to lore. But I'm sure it could be done since GW and CA seem to have a good working relationship. The key is to compare other units againts 1 squad of marines since having 1 spacemarine company being 1 unit would be way too OP. Plus it would be lore breaking to have 1 spacemarine company fight as a block unless the game is TW30K where Spacemarines still fought as Legions and the Primarchs being Legendary Lords. Everything can be scaled up then if it's set in 30K/Horus Heresy. The Emperor could be LL for the Legio Custodes faction...
Total War: Warhammer IV - Cathay, Ind, Nippon, Khuresh (+ Lost Vampire Bloodlines, Monkey kingdom DLC)