Jarvan could be both more satisfying and less broken if you would just change his ult.
His ult is the reason that he's such an issue; you've got a worst case scenario 0 second cc, and a best case scenario 5 second CC. You are then forced to balance his kit around both scenarios, so it's strong enough to be useful in worst case, and weak enough not to be totally overbearing in best case. It's failing at both. The champion is way too strong against people without escapes, and not so much against people with.
Some suggested changes to make it less binary follow.
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Make his R truly impassable terrain, and nerf the duration to 1.5 seconds.
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Take away the terrain change, and make it a 2 second stun, or slightly longer snare.
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Take away the CC completely and make it a nuke.
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Make his R truly impassable terrain, keep the duration, but prevent outside interference from affecting the arena.
All the above would make the spell less binary and allow nerfs and buffs to be more intuitive.