Anyway, to return the topic to proposing more Ratmen for the Ü5 to kill, maim and burn.
Ideas for new Skaven Lords along with some notes for them. 
Egomanical Skaven Grey Seer Apprentice
The idea is that after Rasknitt’s death a con man Grey Seer Apprentice talked his way into leading off a partion of the Pactsworn to follow him on a quest to, well, advance his standing among the Skaven.
The important part is anyway that he uses and expends his followers on getting hold of stuff for and conducting a ritual which he thinks will make him a big and powerful Grey Seer, just in time as the Ü5 arrives - done in a cutscene. But the result is less of a powerful Grey Seer and more of the Vermin in question being infused with a wind of magic and becomes and anchor for it.
Thus the final fight terrain is transformed by the winds of magic into essentially a Weave kind of terrain. And given special powers by the wind of magic in question, the Ü5 will have to take down the manical Skaven before he causes a new magical catastrophy.
The part with this is that it can be varied and randomized for each walkthrough with what wind is going to be encountered at the end fight, which will affect what kind of stuff the Apprentice gets to boost him.
Some Examples
Beast: There could be totems healing the vermin and he is transforming between his cowardly self and a Rat Ogre’s form and so back and forth between these. Perhaps with the “energy blast” that comes with the Tzeentchian Twins on the event to tell when he’s changing forms.
Metal: The Skaven in question gets the bladestorm, becomes armoured, and perhaps has his arms transformed into blades for the kind of duel-wielding attacks that Skarrick does.
Heavens: The Apprentice manically claims to see the future and the whole cosmos, leading him to get the dodge ability and also call down lightning strikes on the Ü5.
The Ratborg
That a Skaven Engineer hired by Clan Fester is close to finishing his masterpiece and sets of with some, or many, Pactsworn to loot and pillage a Dwarf workshop and an Imperial Metal Mage’s residence in order to get his hands on what he needs. In the first two parts the Ü5 drives off the engineer before he can finish his looting and in the third installement he is finally cornered with his warpstone powered mechanical exo-skeleton, ready to take on the Ü5, only to be brought down by our heroes.
Vermin Triumvirate
That three Skaven have joined forces, or rather one big Skaven is pushing two smaller Skaven around to do what he wants, after Rasknitt’s death to claim leadership as mighty warriors. The idea here is that the two smaller Skavens are found killed by the Ü5 at the end of the first two levels, after they have served their purpose for the big Skaven.
The third part is finally confronting the large Skaven who, as an absolute bully-coward, sends in a bunch of Stormvermin before he himself joins the fight with the Ü5. Potentially his third phase is trying to block and avoid the Ü5 while screaming himself shrill for his minions to die for the protection of his craven hide.
Rampaging Skaven Beast Boss
Finally an idea that a Skaven leader, of some sort, has invested a ton to gather up resources to buy one, or more, massive monster from Clan Moulder and so tries to unleash the beast on the Ü5. But the Skaven has taken on more than he can chew and so the beast breaks loose, kills the Skaven leader and then attacks the Ü5.
Given that this fight could be with few or no enemy minions the beast can, or should, be very powerful or perhaps something as dangerous as trio of Rat Ogres or something like that. The main, and hopefully funny one at that, point is that the Skaven has bought himself something he can’t control and it blows up, or punches him, in his face.
