There are almost 50 named Skaven clans in Warhammer Fantasy Lore. There 11 in game and almost all of them are laughably weak. I don't particularly care to play as the Skaven, but I love fighting them and I love the dynamic they bring to the areas of the map that they inhabit.
Add Skab (lives under Nuln) by adding a ruined city somwhere.
Add Scruten in Arnau in the Wasteland.
Add Volkn in the Misty Mountains.
Add Skurvy by placing a new city by Tobaro. Add Skurvy, Skully and Skuttle while you're at it by putting them along the coasts of the badlands, or by adding cities to the Border Princes.
Add Black Chasm and make Eshin the only clan with territory all over the world in its 3 strongholds.
Add Festerlingus in the southlands and put them in the desert in ME.
Add Ektrik and Vrrtkin in the mountains somwhere near Skavenblight so Skryre can have its' thralls. You could add a two city moutain province in Estalia no problem.
The game needs more Skaven, a lot more. I personally feel that the ai should all start with multiple under cities too. Tone down their spread of corruption to counter balance their increased number but the Skaven need some help.