I play this champion a lot.
I mean A LOT.
I mean when you go on lolking and look up who's played him the most this season on NA server, that's me.
He has an intensely satisfying play style, if you're the kind who likes to get up close and personal and there's just something enduringly awesome about his e/q combo.
However, I'll be the first to admit that if the game goes much beyond 40 minutes, you start to become progressively more and more screwed. This isn't a problem necessarily, but you run into the old bruiser's paradox. If I build defensive items, I can't do enough damage to kill the carries and if I build damage items, I can't survive on the front line.
What makes all the tier 1 bruisers so great is that they have something in their kit that scales with a defensive stat. Either armor or HP or something like that, where they are rewarded that little bit extra for building more defense. This is why Jarvan falls off so badly in the late game, I feel, which is a shame, because he's such an incredibly fun champ to play and to watch. Nothing is more satisfying than seeing Cataclysm doing work. You position that thing in a choke point at the right time and it's a game changer. You do it wrong and you throw the game. What could be more fun to watch?!
However, there is the issue of his late game.
Here's the solution I feel.
What's the skill that nobody gets first on Jarvan? W, obviously, because it's really only good in big team fights where you can get a lot of shield and a lot of slow down. It's got a long cooldown and cost a lot of mana so you can't use it for mitigating damage in the lane phase unless you pop it to save yourself from ignite or something or if you're just going all in to get a kill.
If something about his W scaled off his max hp or armor though, all of a sudden you'd have a really good late game ability. So instead of gaining a flat amount of shield for each champion hit with the slow, what if instead that extra amount was based off J4's max hp? Ah ha, suddenly we have a significantly tankier Jarvan in that very very late game stage IF he has been stacking hp (you should by the way. Even without my proposed change to his w, you should stack hp). Jarvan would still be able to do what he does best, e/q right into the middle of a team, slow them all and then split them in half with his ult and live to land another e/q.
Make Jarvan's W scale in some way with his HP.