If they were to rework Mundo, what direction should they take him?
Currently as it stands, Mundo's primary playstyle is building to be this big unkillable asshole and hope the enemy doesn't build grievous wounds. His biggest strength is his biggest weakness and it's what keeps him from actually being good in league today (especially considering that Mundo is one of those stat ball champions that just isn't fair to fight once they get rolling. Not to mention the problem in general with sustain champions).
We can't just outright remove GW (I mean, maybe we can but it'd take more than a Mundo rework to justify it) nor can we just give him immunity because it's the main way of playing against him (cause if he's healing well enough, not even kiting is enough).
I think a primary goal of a Mundo rework would be to gate his sustain through player interaction like other juggernauts such as Darius and Illaoi. The problem here is that with Mundo, regeneration is pretty much what defines him while the other two champions have healing to help them accomplish their main jobs. Darius needs to land his Q if he wants to survive in teamfights long enough to get his ult off for example, Mundo just passively wants to live long enough to outlast people.
Now granted, outlasting people can be a playstyle, but some champions do it with more interaction. Let's take Warwick for example: Warwick has very good sustain, but it is held back by at least two barriers: he has to be at half health for his autos to heal, and he needs to be fighting you to gain health.
How many times have you seen Mundo just run away with his ult, only to come back near full without anything you can do? I've seen it plenty and it makes fighting him incredibly frustrating.
Mundo's kit is very uninteractive imo. His W is just a health costing sunfire cape, his passive will keep healing him no matter what, and his ult is more suited for temporary retreat.
Honestly I don't know if that's ok for Mundo. But he's not a character who is allowed to stay meta for long regardless.
So personally I see two directions they can take Mundo. They're more big picture ideas so sorry for lack of details.
Option 1: make him an actual tank. Give him some hard CC and more engage/peel centric abilities. Personally I'm kinda against this idea as it may clash with how people who play Mundo use Mundo. He's not a protector, and giving him some hard CC may lead to lowering his damage. I'm not sure people want Mundo to be an actual tank, but I wouldn't be completely bothered if he was.
Option 2: keep him as a sustained damage juggernaut, but make his sustain more gated by combat than making it passive. Riot should probably try and encourage Mundo to fight more and stay in the fight in order to gain the benefits of his sustain, rather than just mainly gaining benefits when having enough items to just stand still and not die.
I'm not a Mundo main. I only really played Mundo last night against a team that didn't build much GW and was laughing my ass off at how dumb I ended up becoming. I walked away from most of their team on two separate occasions after killing their squishy carry because my sustain (and once because of a lucky blast cone) kept me alive as I ran.
Wow I rambled. Here's a TLDR.
TL:DR, what should Riot do with Mundo if they were to rework him?