I'm pretty certain that Thanquol will be one of the DLC factions of the End Times, so I wanted to have fun and try to come up with what I think should be two of his faction mechanics.
The paranoia mechanic.
Thanquol only cares about Thanquol. So what happens when a lord or hero in your factions is doing too well? He's obviously plotting to overthrow or kill great Thanquol. He must be dealt with. Introducing the Paranoia mechanic. It's a pannel that tracks your best performing heroes and lords. It assigns "disloyal points" based on how good they are doing on campaign and battle, because obviously, if they do well they do it to make Thanquol look worse.
So what do you do with those pesky traitors? Sabotage-kill! You have actions you can do to sabotage your own armies so that Thanquol himself looks better.
Thanquol's favor.
So, you'll be asking, this is flavorful and all, but it's stupid. Nobody would do that. Wrong! Whenever you do idiotic shit like self sabotage during the campaign you get favor points with the Horned Rat, because he finds Thanquol incredibly amusing. What are favor points? They are a currency that gets spent when Thanquol is in trouble to save his ass.
About to be killed by a superior army after retreating? Favor points get automatically spent and somehow Thanquol is given a Skaven army because bullshit reasons.
Thanquol is killed in battle? No, actually he spends favor points and teleported to his nearest city with a new army.
You've lost all settlements? For a lot of favor points somehow you are given a random city of a Skaven faction.
An orc army is marching towards an unprotected city? Favor points make an army appear.
You get it, don't you? Thanquol is a self-sabotaging idiot that somehow bullshits his way out of consequences.
With enough favor points you could also summon powerful Verminlords and make Thanquol ascend in the hierarchy and things like that.