Wow, this is the FIRST clear answer I got all day :P But one thing still remains unanswered, why have all the jungling champs been more fitted to play in top lane than in the jungle? Is there any primarily jungle champions in development currently? Not expecting any big spoilers mind you :P
It's usually more important to our champ designers to create an experience - get a cohesive kit down, then figure out where it fits into the grand scheme of things. Mages and supports (Mid and botlane) are often times interchangeable. Toplaners and junglers are as well, in a lot of cases. For Vi, i remember having lots of conversations with Gypsylord about her production and not really until the last month and a half did we sit down and say 'okay where do we 100% expect to see her?'
Sometimes we push boundaries, and sometimes we don't. Looking forward to the future, we have some really cool designs that, in a co-ordinated setting, could lend themselves to changing up lane assignments or working in different teamcomps, but i think we've always done that. The meta is the meta, and we don't really decide it. Xypherous i think did this really well with lulu and nautilus specifically, in that Naut is a tank/initiator and Lulu is a support/enabler no matter where they are played. Both champions can be played multiple builds and in multiple roles, but the kit defines what they do, not where they go.
Echoing CerT, having champions 'be good at one role' isn't always the best - Shyvana is my example. I wasn't around at the time, but that champion feels like she was designed to be a jungler through and through - which means her state of balance is binary. 'Is she good at jungling?' If yes, she's probably top-class due to her mechanics. If no, she has no place on a team.
We definitely have a lot of characters like this (usually one build/role) that can be balanced, it just means it's often times trickier than saying 'X mechanic/build is abusive, but at least they can still perform with Y build/in different role to compensate', similar to how Zyra players used her as support after the nerfs to midlane. She was still incredibly potent after the first round of nerfs, but the option for her to play bottom was still there as something alternative she can do if her pew pew gets nerfed.