The current warhammer timeline ends in 2522 with news that Archeon is gathering his forces in the north (Empire Book). The piece below is one possible take on 'what happens next'.
2522: Initially The Storm of Chaos happens roughly as the Loremaster describes it, with chaos advancing up until the walls outside Middenheim. However with Archeon preparing to lay siege he gets news that Grimgor has pulled his orc forces together and is now getting close the city as well. Archeon postpones his assault to avoid his armies being caught in the rear. He leaves part of his forces to defend their base position and takes the rest to counter Grimgor's force.
Never one to avoid battle Grimgor meets him head on. As the two forces line up for battle Grimgor steps forth and challenges Archeon to fight him. The Everchosen can hardly refuse infront of his followers so accepts. It is a fearsome combat but in truth Grimgor's only hope would have been to take Archeon unawares (perhaps with a headbutt?), in an equal contest he is no match for Archeon and is brutally cut down. The orcs stare across the field at the triumphant Chaos Lord, as at the same moment hordes of Chaos Chosen advance towards them. With the death of their leader their morale is frayed and the orc army soon breaks. Only Grimgor's loyal Immortuls make a last stand.
Archeon returns to Middenheim and unleashes his assault, but time is short and before the city can fall to him the Emperor's army arrives on the field. There are several engagements and then the main forces meet in a final battle. Valten challenges Archeon and is greviously wounded, many die getting him to safety. However Archeon cannot be everywhere at once, furthermore he is a target for all the enemy wizards and artillery - with his bodyguard knights reduced to shreds he himself is eventually taken out by a cannon ball while leading a counter-charge against some Reiksguard. Thus passes the Everchosen, slain not in one to one combat with mighty heroes but by a lowly cannon crew.
The aftermath is somewhat more messy for the victors. The recovering Valten is slain by an assasin, meanwhile the chaos forces still need be flushed out from all the positions they captured across the northern provinces. At the same time however the Empire gets some luck, a second chaos army has broken through Peak Pass but it stumbles into Slyvania. Local reports come in of bitter fighting between the chaos horde and forces of undead led by Manfred - ultimately Manfred emerges the winner but such is the effort expended that for the moment he is unable to attack the Empire. The Empire itself recaptures its northern towns but the gruelling campaign and devastation caused has left the Empire financially bankrupt and heavily indebted to their dwarf allies in terms of war-time loans. Even Brettonia is bankrupt from the expense of sending such large forces to aid its northern ally - on top of which is the cost of all the champagne drunk at court in celebration of victory...
Meanwhile in the far east the Hung tribes take advantage of the recent rise in Chaos energy to make their own simultaneous attack upon Cathay. They are only stopped and driven back at great cost. Though they do wipe out the Hobgoblin kingdom along the way.
2523-2527: All this time the skaven have been watching and waiting. When Archeon seemed poised to obliterate The Empire many on the Council of 13 had contemplated they would have no choice but to send their own armies into the battle for fear that Archeon become unstoppable. They were mightily cheered, and relieved, when Archeon fell. With all the other powers having punched themselves out the skaven unleash their own plan by invading Tilea! The Tilean free companies rush home as news comes of their cities being taken one by one. The Tilean Wars drag on for some years with none of the Old World powers in a position to send aid.
At this time Kislev in the north is still under partial chaos-occupation, the main cities have held out but are islands in a sea of anarchy as the chaos warbands carve out their own petty fiefdoms.
2528: Eventually and with most of Tilea already lost Brettonia is able to declare an Erranty War. However by this point the skaven occupiers are well dug in (excuse the pun) and with their superior technologies and artillery they are more than a match for Brettonia in the ensuing game of sieges. The Brettonians make little headway and eventually winter sets in. Their army, decimated by disease and dysentry, is forced to make its withdrawl. Skaven for the first time own a kingdom on the surface, consequently there is a mass upsurge in Skaven piracy. Among the Council of 13 there is even talk of building a fleet to invade Ulthuan, planning begins though this ambition remains some way off in its potential fulfilment.