When I replay ME3 with the DLC, my plan is this:
Omega comes after the Cerberus coup. Omega's biggest problem is that it's so far removed from the rest of the Reaper war that you're left wondering why Shepard even bothered to tag along. By placing it after the coup, it can be justified as a retaliatory strike intended to weaken Cerberus so they can't make a direct attack on the Council races again.
Citadel comes after Rannoch. The Normandy has just spent several days or weeks on the far side of the galaxy attempting to resolve the Geth/Quarian conflict, and after that the ship needs repairs and the crew earned some shore leave. It also works as the final "fluff" piece before heading into the dark final act, sort of like DS9's Badda-bing, Badda-bang
Leviathan comes after Citadel and before Thessia. The reveal at the end of Leviathan is so huge that it doesn't make sense for nobody to react to it in the rest of the game, so it needs to be placed as late as possible. But Thessia is the start of the endgame, from there it's a linear path against the clock to Earth.
Based on what they've said so far, what little they've said so far, I don't see them trying to go bigger. Trying to do so would IMO, be a mistake. It would be like the ME version of Babylon 5 and the failed Legend of the Rangers series - "What, we've beaten the ancient powerful enemy, the Shadows, but there's someone even MORE ancient and powerful out there?" - Erm, no.
Well, I've not been following ME news of late so I don't know much of what they've said about the next game in the series. Whatever they do, the next ME game will be an anticlimax after the spectacle of the Reaper war, and I hope they acknowledge that and choose to tell a smaller story instead of fighting against inevitability and trying to tell an even more epic story.
And yes, The Legend of the Rangers was in the forefront of my mind when I originally posted that.