@Riot Regarding Offtank Caster Itemization/Keystones
In the past there has been a reasonable level of versatility in playing several AP champions more as an offtank type of role. Wading into fights more for sustained damage/CC rather than in your face burst or pure support item builds they might be traditionally played with.

Some champions obviously pull it off fine (
), but it feels like it has to be almost explicitly stated in their kit for it to be viable. What is perplexing, and maybe Swain isn't the best example since he is currently so strong- but they manage to be tanky and do a lot damage at the same time. Other tanky AP champions (even though not exactly meta) still can have reasonable level of success building an AP/tank mix and relying on their innate tankiness to make them a viable offtank.
With most of the above (first list of) champs, however, if they build maybe 1-2 'standard' items and the rest as HP/Arm/MR to shore up their defenses they simply end up as failures at both being an offtank AND at their traditional damage or support roles. I understand why you might be hesitant, especially after the tank item nerf from a while back- builds like tanky machinegun Lulu were incredibly powerful back in the day. That said, it seems like if you had an item or keystone specifically designed to accommodate this play-style you could more reasonably keep it from getting out of control while returning the option back to players who so enjoyed it.
Thanks for reading and I very much hope you wouldn't mind briefly commenting on your view of the caster offtank playstyle!