I've had this idea going through my head for a while now, so nothing of it is fully set in stone. But what I've thought of it so far is this.
It is set after the end of the Sailor Stars anime, and features things that were exclusive to the manga and PGSM!
Following the battle between Sailor Moon and the Chaos-possessed Sailor Galaxia, Chaos had been expelled from Galaxia's body and everyone who had died from his attack had been revived. The Sailor Stalights and Princess Kakyuu returned to Kinmoku, and the ten members of the Sailor Team can finally return to their normal lives.
However, all is not what it seems. Both Rei and Michiru are receiving strong feelings about something that is going to happen (while observing her fire, Rei is horrified as the fire burns up so greatly that it actually breaks the firebox that its in, and Michiru's Deep Aqua Mirror actually cracks on its own, to her own shock).
Before he was defeated, Chaos had anticipated the possibility of his defeat. Following his demise, three dark figures appeared in space. Then they vanished, reappearing in an unknown location that is their hideout. In this place, they are fully concealed from any detection by the Outers (remember how easily the Outers detected the presence of the Death Busters?). The three discuss how their master Chaos has fallen, and that it has become their mission to ensure that his revival is secure. In addition to that, they are to find a way to accelerate his ressurrection as soon as possible, as well as locate the ones responsible for his defeat and make them suffer (but not kill them, as these three are in the belief that that honor should be given to Chaos once he is revived).
The three figures are the cool, cunning, calculating, and honorable leader
Tartarus (he looks kinda like Marvel's
Mister Sinister in blue armor); the ill-tempered, aggressive
Erebus (he has long fiery red hair, a small pointed red goatee, and wears dark gray armor); and the mysterious and manipulative
Nyx (she wears long black robes and a black veil over her face, revealing only her eyes, loking kinda like Marvel's
Dust character). Together, these fearsome three form the mighty
Primordial Imperium.
The plot involves the Sailor Senshi fighting against the Imperium. Unlike the previous villains, these three actually fight against the Senshi themselves, alongside their monsters-of-the-day (whose name I can't think of yet). Unlike previous MOTDs, their MOTDs don't get destroyed every "episode". They each last for at least two episodes, as they are smart enough to know when to retreat so they don't get killed as often like the past MOTDs did.
Near the beginning, Tartarus reveals his presence to the Senshi and during their initial encounter with him, the Inner Senshi face him directly, but he proves that he is too powerful for them to defeat, and to make matters worse for them, Tartarus shoots out energy beams at Sailor Moon, and then at the four Guardian Senshi, that grab each of them and start draining away their Sailor Senshi powers. He intended to strip them of their powers completely, but an interference from Tuxedo Kamen and the Outer Senshi prevents him from draining them completely. When he retreats, he takes the powers that he sucked out of them along with him. The five Inner Senshi are still alive but have been seriously depowered back to their original Sailor Senshi powers from Classic.
Because of this, the Outers, who were not present at the energy-draining, take a more prominent role (to make up for their lacking roles in Sailor Stars), and Luna works to give the Inners new Senshi powers to substitute for the ones that they lost. The result is their PGSM forms and abilities (Moon also get a new weapon in the form of the Moonlight Stick, which she notices is similar to her old Moon Stick).
There's more that happens that I wish to talk about, but right now, I need a break.