What should Trundle's gameplay theme be?
I don't think it will shock anyone in the room to say that Trundle's design is incredibly dated, suffering from both a lack of complexity and too small and weak a niche to properly operate.
In competitive play he's a hard counter to tanks like Sejuani or anyone with Stoneplate and temporary defensive buffs. However the rest of his gameplay fantasy as an Anti-Tank stat stealer really falls flat because of how easy it is to kite him, or simply get outside of his W.
Speaking of his W however, he does have a secondary aspect of zone control to him that I don't think many people recognize and respect, even despite the fact that it's a huge pool of his power and frankly speaking the most interesting part of his kit in terms of gameplay (as opposed to theoretical number crunching from his stat stealing). One of the big problems I see with his W though is that it puts far too much emphasis on Trundle being strong while in it, dumpstering Trundle's effectiveness outside of his W and not actually giving an incentive for the opponent to get away from it outside of getting away from Trundle, which is what they should be doing anyway. In other words, it poses bigger restrictions on Trundle than it does on the opponent.
Considering the slightly more modernized class system, it's easy to describe Trundle as a Juggernaut, and his current iteration has two themes of stat stealing and zone control. Should Trundle ever get reworked, I would love it if one or both of these aspects are allowed to shine in order to maintain his current identity as "Trundle".
The following is not a suggestion, but rather an example to get an idea across. Imagine if Trundle's W and R were combined into a single Ultimate ability, something along the lines of "Trundle gains tremendous tenacity and movement speed and steals x stats from all enemies standing in this zone". It would last far longer than Mordekaiser's Ultimate, say 10-12 seconds, but it has a similar AOE to his current W. Furthermore, outside of his E, Trundle won't have many ways to actually lock somebody down inside his Ultimate. While it is intended to be a "super mode" ultimate in the same vein as Mordekaiser or Illaoi's Ultimates, its purpose will be more specifically for drawing a large "do not enter" AOE on the ground, punishable with your stats getting stolen and death. It would open up an entire slot to help Trundle's basic abilities flow better, and more importantly, it gives Trundle a strategic niche that is similar to his current one while also not restricting him to be just a Sejuani counter. Again, I don't mean this as a suggestion, but it is the idea I would want to see a modernized Trundle take; current Trundle can be defined largely by stat stealing and zone control, why not combine these elements so that Trundle becomes the zone control Juggernaut who overpowers you by stealing all of your stats?
Anyway this is only my opinion on both our current iteration of Trundle and the direction I would love to see taken with him in the future. What do you think defines Trundle's kit, what should be kept if he ever gets reworked, and wh