Fix subbing please.
Honestly SKT was at a gigantic advantage in that best of 5s. Not only because they're some of the very best players in the world but because when they're tired, they can literally just tap out and have someone take their spot. Faker/Easyhoon did it last year and Bengi/Blank did it this year.
Do you remember game three where Bengi was fatigued and was consistently being outsmited by Ambition? Bengi could get that swap, and for a full game he got to rest and come in during game 5. And what do ya know? He absolutely crushed SSG. SSG didn't have that privilege.
I think that having subs is fair because it allows you to try more team oriented play and see who meshes well with who, but having a sub swap in mid-match creates a very powerful advantage where you're rested, have had 2-3 games to scout your enemy and know what they're running this match, etc. That's why I'm proposing that unless the player incurs some sort of serious injury (which isn't likely anyways because this is League, not football), the 5 players they pick to represent them have to be locked for the match.
This means that in all spring/summer split, you can use whoever you like. Hell if they're on your sub roster you can bring in Trick, Pewdiepie, and the ghost of Dunkey every other game for all I care. Just in best of 3s and best of 5s whoever you start with you should stick with unless serious injury is incurred.