Giving Jarvan AOE damage on his ult; Do you even play your own stupid game Rito?
Congratulations Riot. You have given damage Jarvan a buff.
FUCKING NOBODY LIKES AD JARVAN!
People don't like having him on their team because he's a less good Zed. He assassinates a single target pretty well and he can split push pretty well, but the enemy only has to get 1 item and he's worthless. In Zed's case it's qss, in Jarvan's case it's any amount of hp+ having flash. Woopty shit. You haven't changed that about him.
People don't like to play AGAINST Ad Jarvan either for the same reason they ban Zed. Zed is incredibly manageable to a well co-ordinated team who can just accept that a few mid game deaths will happen so keep scaling and keep turrets alive and keep trading objectives and win the game late, but the fact is, nobody likes getting killed 1v1 when it feels like there's nothing you can do about it. That's what AD Jarvan does. He flag and Drags onto your face, ults you, hydra's you and you're dead. It all takes a fraction of a second to execute, it's very EASY to execute, especially against low mobility champions and there are a whole set of champions who have absolutely no defense against it. If you want to walk up and cs against an AD Jarvan, he can kill you. That's not fun to have done to you.
The thing that makes Jarvan a great champion... The thing that SHOULD make him a great champion is his ability to start fights, his ability to get the hell into the fight first and make a path for his allies to follow or hold the enemy down long enough for his team to get in there with him.
Here's what I suggest. Either, give his flag a Janna passive component, where allies get bonus movement speed if they are travelling towards the flag (allies who are not Jarvan himself) or, give enemies a small slow if they try to move AWAY from it. Just a very minor 5-10% based on skill level. Something subtle but something useful.
If you don't like that idea, how about when you cancel his cataclysm early, it creates a zone of rough terrain around the circumference where the wall was destroyed which slows the movements of enemies trying to walk over it. Again, 5-10-15% per level. And if you don't cancel the ult, deal a bit of damage to enemies still in the cataclysm so long as Jarvan remains in it as well. You could even reduce the initial damage and put more of it on the finishing blow. And for christ sake don't scale that damage with AD. Scale it with HP or something. That would reward Jarvan for building HP again, a thing which has been almost pointless ever since you took the passive armor scaling off his E.
Stonewall had some pretty good ideas about things you could implement with his Flag in a rework too and none of those were focused around damage either.
I know how you coded his ult, I KNOW this is a thing that you CAN implement without radically changing the code. As it is right now, most of the time the correct thing to do with AD Jarvan's ulti is to break the wall imediately before vayne or poppy can use it against you. How cool would it be if you actually had a meaningful decision to make as to whether you wanted to keep that wall up for more damage or break it down for more chase potential and team follow up? But no. You do the predictable and boring thing and just give him more damage and, if it turns out to be quite strong, you will put him back on the auto ban list.
Again, I ask rhetorically; Do you even play your own stupid game? Sometimes I doubt it.