Here comes the HERESY...To me, the situation in the Reach is a fine example of "there is no road to victory", and that means that they can keep writing supplements, and not have to crank out a whole new system, or setting within it.
For the Tyranids, I don't know if the worlds they touch can ever really be retaken. I surmise that they lace the place with their own bits, sort of like Orks with their spores, and recurring resurgences of little Nids will be the plan of the day forever. Short of reducing the planet to an ash-covered surface, leaving nothing of value save mineral deposits, getting rid of the Nids seems unlikely. Also, while I don't know how big the entirety of HF Dagon is, if it is filling much of over a third of a sector, that's a lot of Nids to have to deal with; Nids who have a much easier time mass-producing their best stuff, something the Imperium, even with a million worlds, always seems to be terrible at. Anyway, they will probably keep coming back, and if there is only one Hive Mind, and Dagon got out there, somehow, nothing stops the HM from reinforcing the Hive Fleet with more of its limitless resources. Human-held worlds could have Gaunt infestations, or Genestealer Cults forever. Likely, you would have to commit a total space fleet, and destroy every planet, just to cut them off, which begs the question why you said you wanted that area? That loses you a third of the sector.
Onto the Chaos. They have much of the same stuff, and a considerably easier time getting their stuff from point A to point "your tail pipe", so they will continue to be a real threat. If worst came to worst, they could just pull local forces back into the Immaterium, reinforcing other areas, and guerilla warfare the Imperium forever. You might not hold a world for a whole year, and then another goes off the grid, and on. Also, they have Daemon shock troops, so that can be hard to handle.
Lastly, the Tau. While I see them as the least threat, overall, they could be a problem. In a way Chaos can't, the Tau could draw off numerous allies, and make them enemies. While I see the Imperium as a good substitution for Darkseid, populated by people who can't even comprehend what it might mean to rebel against his word; he's a God, after all, and even if the xenos say "you can still worship Him", He said "death to all xenos", so who cares what they say?, there still would be losses, and fighting them leaves pressure off the Chaos and Tyranid flanks, meaning that those two get stronger. The Tau also just have better gear, somehow, and a lot more of it, to pick at their foes with. It would help if the Imperium wasn't so inward-focused that they could actually ally with the Tau, and kill the other foes, but they won't. Best option then might be a powerful propaganda campaign, painting the Tau as more than just another hated xenos. Broadcast how they might be sterilizing Human converts, or conducting terrible experiments on them. Say they are violating Human females, trying to make a superior race between the two, whatever it takes. Just help the already indoctrinated people have all the more reason to hate particularly the Tau, and the convert problem should lessen.
And then we get to the Imperium, filled with hatred and egocentrism, overtaxed and overextended, just really too big for its own good, and still trying to grow larger. For some reason, they have always been painted as this lumbering behemoth that, even with a million worlds, has forgotten half of the cool stuff it ever had, and never being able to crank out enough of its cool stuff left to put where its mouth is, even while lesser races (Tau), build comparable things fast, and in mass quantities, with less than half of the available resources. Add to that theat the Imperium is busy with X number other Crusades, a thousand little wars, and on; they can't reall afford this three-front additional front, and can't send too much more into it, without costing them something else, elsewhere, as the galaxy is filled to overflow with things that want to destroy the Imperium, both inside and out (and if you look at how they treat those inside and out, is it really a wonder why?) It's not really a wonder, compared to Choas (our source is beyond your reprisals), the Tau (we put your monumental building prowess to shame both in scale and quality), and the Tyranids (we build as we go, whatever we need, and cannot be reasoned with), that the Imperium is having trouble in the Reach.
A solution: if it were possible, I know what sort of idea I like. On Cadia, there are these ancient structures, of Necron manufacture, that deaden the Warp within a certain range. I believe that these are part of what makes the Cadian Gate. There are supposed to be more of these, as the Necrons, prior to their Tomb King oops-grade, were going to use these to deaden the Warp throughout most of the Materiam. It could be of some limited use to use these to slow two of three enemies, erecting these structures to block Chaos, and to disorient the Nids. While I know that the Imperium forces also depend on the Warp for simple communication, and long-distance travel, these could work in certain key areas, giving the Imperium vital footholds, and Cadia works fine, so maybe these could, too. If you could force either Chaos or the Nids to have to fight without their Warp-connection, they could be severely weakened, giving the Imperium a prayer against their forces. If nothing else, they could just pull out of the Reach (probably the best solution, with failing resources, and two endless foes), and erect a sphere of null cubes around the Jericho-Maw Gate, on the Maw end. These would force the Gate back into slumber, and close the road, making any incursionary forces have to take the long way, or hit the other side of the Imperium. I'm not sure what was so great in the Reach that the Imperium wanted it back, beyond the whole "everything is ours, by His Divine Decree!" schtick, but if the Imperium can't commit more forces, and maybe some more esoteric forces (Rogue Traders, for instance), or make some allies, I don't think that they can take the region. A Hive Fleet wins on its own, and there are two other forces bothering, too. Unless the Nids could be directed at Chaos, and their Shadow in the Warp could cripple the Ruinous Ones' minions, forcing the two to throw everything at each other, until one of them was eradicated, leaving the other winded and depleted, either could stand forever, due to better "supply lines", and superior quantities of better stuff. The Tau aren't as strong, but they don't have to fight as much.
Beyond the Null Cubes (an unlikely solution), the best bet might be to just suck it up, and get the Nids to hit Chaos (probably easier than getting Chaos to hit Nids), the Tau to hid the Nids, and try to draw in a few other forces that aren't as big. If the Nids had to fend off Chaos, Tau, and Orks, even they would be hard-pressed to win, and would leave the surviving Tau, Chaos, and Ork forces weakened, or dead. Then, the Imperium wipes up what's left, and wins. Good luck with that, though, too. Maybe, if all are lucky, the Necrons will wake up out there, and take down one or two threats, other than the Imperium. They would relentlessly pummel Chaos and Nids, and are much slower to spread, after they win. If they killed enough stuff, they might even just go back to sleep, once the pop. counter falls below a sustainable level.