My personal suggestion list for this game:
1: Add a morale and suppression system for units ala a mix of Dawn of War 1 and 2. If a Squad is say, comprised of five troops, and 3-4 of them die, then their morale would be shaken to shattered in no time. They would lose effectiveness in combat, no matter the buffs or "doctrine" (see point 7) enabled. Likewise, the suppression system (see point 5) from Dawn of War 2 enabled a variety of tactical depth to that game, where this game has none. Couple this with the cover system in Dawn of War 2, and you could create some truly glorious choke points or secure flanks with heights, buildings to garrison, or traps to place.
2: A Dawn of War 1 Style Squad upgrade system: i.e. kit out your Squad with Sergeants (with an attachable Squad Standard), Plasma, Heavy Bolters, Flamers etc based on what's going on. Bring back the tactical depth to unit layouts basically.
3: Custom keybinding options. It's honestly unforgivable to not have them in a self proclaimed RTS.
4: Actual base building. Buildable walls, turrets, more buildings that have more upgrade options and units ala a mix of Dawn of War 1 and 2. So instead of having two buildings that produce Tactical Squads, or two separate buildings that build Scouts and Devastators, see Dawn of War 1 (and my 2nd point) that gave you a base Squad, and you upgraded them from there, after of course, researching the appropriate upgrades from other buildings. Actual defensive lines would be great, placing mines, turrets, walls, etc to create choke points or secure flank areas to drive the enemy into your desire kill zones.
Adding the cover and suppression system, along with the firing arc mechanic from Dawn of War 2. Getting rid of the pointless bubble cover and invisible patches, would be a great step in the right direction. Bring back being able to garrison buildings, and make unit placement important. In my opinion there's zero tactical depth when it comes to unit placement. They just stand out in the open and get killed off while trying to protect a point, or they just stand there and get shot at without firing back. Which reminds me, a stance toggle from Dawn of War 1 would be great too. Firing arcs would be a good way of telling a unit "face this way and shoot at any enemies in your cone of fire". Couple this with a stance option, and there will be no doubt that a Squad wouldn't just stand there useless as the enemy rushes them for an easy capture of ground.
All future faction DLCs have to be free. I mean, let's be honest here. Dawn of War 3 has lost a very large amount of its player base, from an all time high of what? 20,000+ players, to a regular active base of around 1,000+ players, if that. I think one of the biggest reasons for this, is the lack of factions, especially iconic ones like Chaos or Imperial Guard/Astra Militarum. I also think probably one of the only ways to get these players back, is to offer them actual incentive to play again. I've honestly stopped playing because it's not the Dawn of War game I wanted, but, I might take another look at it if the Imperial Guard/Astra Militarum or Tyranids were added.
Get rid of doctrines completely, they're pointless and a waste of time. Seriously, in my opinion they offer little to no tactical depth or customization whatsoever. In my experience, nine times out of ten I've only used the same three doctrines every single time I've played. Also, the fact that in order to get a "good" doctrine means that I have to play a Hero I don't like, in order to get a doctrine I need, means I'm forced to play the game in a way I don't want to. Doctrines replace reliance on solid tactical sense with reliance on a few "boosts" that may or may not help you in a match.
Make it so Heroes are like how they were in Dawn of War 1, where you could build them at the appropriate building after completing an upgrade, and be able to attach them to a Squad, so the Squad greatly benefits from their buffs. You can still micro units with attached heroes, though they wouldn't be as "standalone" as they are now.
Leave Requisition Points as they are, but Power Generators should only be buildable near buildings you've created, ala Dawn of War 1.
Last Stand gamemode. We saw that it is somewhat possible with the last mission in the campaign. Might as well create that mode for multiplayer. I would also suggest that as an additional option to having a Hero unit, you could take a customizable Veteran Squad to use in said Last Stand gamemode.
I'm fully aware that by this stage in the game, radically altering or changing the already in place game mechanics is going to take months of work and money that the devs probably don't have. However, I think these changes can at least bring back half of those players who have long since left this game, because for myself and many others, this isn't the Dawn of War game we wanted. That being said though, something needs to happen in order to bring people back.