I don't think TW's 'battle style' would really fit 40K, or any period of history after line battles started to decline, though it could work. In regards to the strategy/political phase the usual TW style could be made to fit, but when it comes to battles, if large scale is what you're after, something more akin to Wargame/Steel Division (with melee) would probably be better. And then throw in some Battlefleet Gothic style space battles I guess, but by this point such a game feels like it's entering "Hire this man!" territory.
Funny you mention
Steel Division, ideally, I'd like an
SD2 style of game that has a better sense of scale in with squad based tactics and the concept of rolling battles. AFAIK, though it wasn't received especially well for one reason or another, but the core gameplay loop ideas are definitely more suited to the WH40K setting though, IMO. You may find me a bit too much of a TW fanboy, even if I realise the game series and its engines have their flaws and limitations. Still, regarding the suitability of the archetypal battle gameplay in TW games:
With less units doing line formations you'd just adapt the skirmish formations from R:TW, TW:WH etc. for most factions and what you'd get would functionally be the same as how units form up on the TT. All you'd need after that are more kinds of terrain and maybe some sort of deformable terrain mechanic (for craters, large units smashing obstacles etc.).
This was my initial thought on it (still is, in fairness, though with a bit of expansion).
R:TW was pretty much the only game with irregular formations (Skavenslaves in
TW:WH2 have it, too, but it's still obviously meant to be, broadly speaking, a 'line formation'). This can be adapted to a more modern warfare feel (with varying levels of acceptable breaks from reality what with non-transport based Fast Attack infantry) with slightly decreased unit entity elasticity (AFAIK, unit cohesion in multi-entity units is based on rubber-banding around the unit's centre of mass) and increased spacing.
Total War's mechanics are based around line infantry, cavalry formations, and simple war machines. It's not set up to simulate combined arms warfare. The latest war I think it could reliably emulate is the American Civil War. At a push it could probably do WW1 style combat, but anything similar to WW2 or newer wouldn't really function well as a Total War game.
While we can agree to disagree, I'd point out that because of the nature of trench warfare, the sheer scale of the majority of battles and the benefit of hindsight makes even
TW:WWI an impractical concept to pull off without a massive overhaul of the engine and gameplay mechanics (again,
SD2 gameplay seems more suited to it). If you've seen
The Great War mod for
Napoleon, you'd see how difficult it is to have an authentic portrayal of WWI warfare in the TW mould of games. WH40K, on the other hand, is an over the top grimdark setting where 'tactics' and 'strategy' and informed aspects of in-universe warfare. Line combat essentially still exists (Dropsite Massacre & anything the Mordians do, indeed a fair few of the IG/AM doctrines are based on linear tactics), specialist melee units (aforementioned Fast Attack units are invariably melee focussed, as are a large proportion/small majority of HQ choices) feature heavily, psykers and copious special abilities can render any sort of hard cover useless, hit n run bike squadrons are essentially mounted carabiniers and charging with them into melee will happen, and some races just aren't known for good gunnery. I mean, Space Marines are the archetypal jack of all trades and marketed as such for the tabletop hobby, so while I concede that any TW game adaptation will be ranged focussed, expect melee to nonetheless feature heavily.
Last thing, I'll betray my biases by pointedly stating (apologies for being facetious) that the Wars of German Unification would, owing to comparable technological level, be the latest war a TW game could emulate.
You'd probably also have to deal with transport vehicles, since you can't cram a hundred guys into 1 APC
With the new WAAAAAAGH! mechanics introduced with the last update, this is solvable, I think, by a tweak to the secondary army panel. Purchase unit, then purchase its transport upgrade which appears in the secondary panel (needs more thought, obviously, because of multiple transport options a lot of the time). Whatever damage/replenishment the parent unit takes translates to more or fewer transports and done (a bit inelegant, but hey, how do you rationalise replenishment in the first place?(!)).
2. Scale. Do you go for 1 planet, a sub system or something else? One planet is likely to feel confined, but doing a large sub-system or sub-sector can easily get too big and become confusing to navigate around.
A single star system is what some agree is a suitable setting as it allows for space battles without being too overwhelming in scale it basically becomes the realm of Paradox/Stellaris. If, however, as was pointed out to me earlier that development resources may not allow for including space battle gameplay (possible and rather reasonable, if disappointing), a large planetary conflict a la
Dawn of War: Dark Crusade or a handwaved transportation mechanic a la
Soulstorm could be done instead and that would resolve (most) things handily.
The really hard part would probably be making a lot of unique faction mechanics, as the races are far less distinct than their fantasy counter-parts and the total war aspect of 40k is up-played far more.
I think that only really applies to the Imperium factions and mostly because GEEDUBS just won't stop milking it. The xenos races alongside the core Imperial factions have the same diversity as in WHFB, so I don't think that's going to be much of a problem.
I think it could work if you think of planet as continent and space in between as ocean. You'd just go from one planet to another in ships (with some boarding action possible) and when you'd get there you'd land in pre selected area.
Or the local port (if they segregate regional main settlements from resource buildings)/port-settlement (or not) if you control it, obviously (unit transfer like in
Empire/
Napoleon). Not sure what you mean by 'pre-selected area', but are you thinking of clearly marked (if not necessarily labelled) 'suitable landing zones' analogous to those stretches of beaches in
TW:WH2?
