I've been spamming Swain, two suggestions for some QoL changes
I love the new Swain, and I think he's going to be a mainstay for my mid lane roster. That being said, there are two gameplay issues that piss me off so bad that if they were on nearly any character who isn't brand new and fresh I'd probably tilt out of playing them.
1- Sometimes hits E hits them on the way out, but not on the way back in. What I mean by this is unlike Ahri orb or Sivir boomerang which will proc twice almost instantly if it hits someone near the max range, Swain's E for whatever reason only hits them on the way out but not back, meaning you don't get the root, his passive, the life from the soul from using passive, and more damage. It really, really blows. I think the retracting E has a smaller hitbox or something but I think they should make it so if it hits someone max range going out, it should hit coming back, it only makes sense.
2- Swain's ult goes on full cooldown even if he dies before it grants health. Remember Evelynn 2.0's ult that gave a huge shield when you ulted someone? Remember how it used to have a slight time like Triumph where you ulted and didn't get the shield, dying as a result and it sucked? Swain's ult is worse than that, because it doesn't even do damage first. Seriously, if Swain dies activating ult, make it not go on CD. It's such a long cd and doesn't go down with rank.
getting CDR, more so if your running inspiration for stopwatch and cosmic. I originally thought 
was potentially an absolute core item on him but plenty of times just Pred MS boost is enough to set up kills so I'm fkin around with some more item builds. Also pred rune damage tends to end up being similar to the Dark harvest damage when I was testing that rune.
vs some comps as his auto range is pretty solid and the stats are quite useful for a defensive item slot. on that note Klepto is pretty easy to farm with him and there plenty of inspiration runes that are solid. Glacial is semi usefull for getting him into better Q range or keeping folks in his R but the active item slows aspects fairly underused so eh. Spell books never a bad move.