After playing skaven quite a bit recently i have a small buff suggestion that might help overall performance without being a major game play change.
I will first mention that i actually feel most skaven units are quite decent for their cost, not all but most but to be honest i have not used them all regularly so it very well might be that i'm simply not experienced enough with some of them to determine what specific buffs they do deserve, but one thing thats very common is the skaven LD and issues with undead factions as a whole, this mainly comes down to ld but staying within skaven theme its not appropriate to buff the units ld, so it came down to how do you buff ld without buffing the units ld, and here is my idea.
"Strength in numbers" should add +1LD for every non expandable unit within 55m of the unit capped at +5. This would mean most skaven units in the roster would get about +3 to +5 LD but any isolated units would not benefit from this bonus. With a cap at 5 this should be an anti blobing measure.
Now there is one specific unit that i do think needs a direct buff and that is stormvermin (halberds) - now i might get some slack for it BUT i do think their performance vs cav and large is fine, where i do think they lack badly is how much they underperform vs other units compared to sword and shield version, this might be because there is no great weapon variant, this is problematic in my view because at such cost skaven who guess opponents army composition incorrectly are at a great disadvantage in their elite section.
My suggestion for Stormvermin Halberds is -4 Bonus v Large, +3MA, +3 AP Weapon Strengths, this should keep their performance vs cav about the same level but should be quite a boost vs infantry.
I do think those two changes would help skaven quite a big deal overall.