Swain might be a support now.
His quick AA animation, and long AA range give him the ability to use the AP support item efficiently for harass. His E is relatively easy to land on bottom laners, with the exception of maybe Vayne. His W works for sandwiching enemies into a gank(Walk into my W, take damage+get slowed, or our jungler reaches you sooner). His Q, pretty useless, just harass for when you land E.
Also, playing as support doesn't leave u mana starved, because u aren't using Q to CS. Youre only using Q off of landing E. With this in mind, it gives you more mana to use E more often, which generally gets you 5stacks quickly, and youre never in a position where you do some 0-3 stacks ultimate.
Glacial augment+Hextech-GLP800 as 2nd item.
His scaling is so abysmal that there really isn't much point opting to be a mid lane mage(Hes terrible at being a mage), vs being a tanky utility robot.
Ive had better luck playing him as support than anywhere else so far. He feels right in the bottom lane to me.
as my first major item, and Swain's W plus a sight stone meant we won the vision game. Ultimately we won, but I hit like a wet noodle and died like one too, but my CC and vision made up for it I think. I think if I had the chance to build Liandrys or Crystal scepter I wouldn't have been so easy to kill and might have done respectable damage, but the game ended too early to see late game Swain support.