First just wanna say that the last time i had this much fun on a singleplayer grand strategy campaign was probably since Medieval 2 Total War. Im on turn 120 on the imperium campaign and its been awesome. I really like the fact that you are on the edge deciding which systems to defend with your limited fleets, and i specially like that you can create specific chapter fleets. Im getting quite attached to Demetrius, commander of my imperial fists fleet. All in all im loving the game, the campaign such as in Dawn of War Dark Crusade aint got nothing compared to this one imo!
Anyway, an actual campaign where you can pick between all the independent factions, either scaled through the whole galaxy, or say, just segmentum ultima (so tau doesnt gets excluded) or segmentum obscurus, would be awesome. One option is to have, as in the imperium campaign, the Imperium being one faction and if you play as them you play as all the space marine chapters, the admech and the imperium navy. Another interesting option is using a "vassal" system, where you can play as the Imperium Navy, but the Admech and all the 9 space marine chapters are "vassals", total war style, to the imperium, but they are independent as in you can play specifically as that faction, while the imperium would be played by the AI in that case. Conquering a system means you get the option to choose it to be a recruitment planet for the chapter or what not, or leave it for the imperium.
Playing as a small cog in the greater imperium and pushing your chapter to the limit to save the imperium (the AI) would be quite interesting.
-So, even a "diplomacy system" would be useful. Not for some weird alliances happening, but just so that all the independent chaos legions at least start with a semblance of one purpose, and so all the eldar craftworlds (each one a single faction as well) dont go to war against each other. With orks and tyranids (iirc i read something about different hive fleets fighting each other) of course that could be ignored and all out war happen.
-An "unrest" system, tied with an infiltration feature, can be a good idea as well. Say, if you are chaos or tyranids, you use some resources or points to infiltrate a system, creating a genestealer cult for example if succesful. Then for the Imperium player the income from the planet may diminish, the unrest rise, and if not checked the whole planet may rebel.
-On the zoomed out map where you see the sectors, you should be able to see the disposition of your fleets, and a planet that is under an invasion or attack in the next few turns should be highlighted. (not just the symbol of invasion appearing next to the whole sector as you move the mouse over it, as it is now).
-A system of garrisons in systems. If you use a bunch of resources WITH an upkeep, and create a garrison in the system, if under attack and the space battle lost, the garrison keeps the system from falling in the same turn, so a "siege" happens for, say, one, two or three turns, depending on the level of the garrison. Once the three turns are over and the enemy fleet is still in the system, the system falls to them. That gives time for a counter attack from a relief fleet.
-No arbitrary limits to the number of fleets you can field. It should be a flexible limit imposed by your income and, perhaps "manpower, depending on the faction". The imperium, starting with the biggest number of provinces, should be besieged by all sides, so though they might have a gigantic fleet, either they are stretched thin, or they focus on one enemy and lose provinces everywhere else.
-No arbitrary limits below the maximum game limit of the number of ships you bring in a battle. If the limit of the whole game, say, is 1600 points, then if you have a fleet worth of 1600 points in turn 1, you use all that from the start of the battle. You SHOULD be outnumbered when you are indeed so, and vice versa.
-On a battle where you have two fleets and a higher number of points above the game limit, the order of reinforcements should be picked by you, so you choose which ship will be the first to arrive once there is room, and so on.
-Perhaps a deeper trait system on the fleet admirals, just some general stuff that gives a very slight positive and/OR NEGATIVE effects on the fleet he commands, but not limited to just 2 stuff. Can be simply randomized in fact, and not up to the player to choose the best. That way when an actual good admiral appears its quite helpful and immersive. Similar to the early total war games, M2TW and RTW, and their mods such as Europa Barbarorum, one character could have a whole twenty traits on him.
-Unique stuff per faction, side goals, objectives, the way resource gathering goes, how they deal with the sectors, etc etc. Basically what you guys already did to the tyranids and necrons, but now to all the rest as well.
Im probably hoping too much. Ive read that another grand campaign is set to be released later on. I sure hope its similar to total war in style, where you can play 800 turns and still keep going (as i just did on my europa barbarorum 2 campaign). The player sets the overall goal of the campaign, and there should be so much systems and sectors, perhaps 200 systems, that the road to complete domination should be a long one, but a very fun one where you immerse yourself in the plight and glory of your forces.