During that 15 second starting period of the match. I believe players should be allowed to swap their summoner spells. It would not affect the game, you could announce it like the spell-book mastery does, and i believe it would save a lot of games in low to even high elo. 
Warlord Rhinark2 votesThe problem with this, is that you will always be able to counterpick what your opponent's summoner spell is.
Need Gold 4 Tent2 votesYou: "Damn, he counters me. Let me jungle" swaps tp with smite, jungler takes tp
Other team: "Oh they swapping. I'm going to go top and screw him over* top takes smite/jungler takes ignite*
You: "Fking ignite. I have to take ignite too" top swaps tp with ignite
Your bot: "Ah great, no tp plays from top lane. We'll play aggro" swaps ignite for exaust, heal for ignite
etc etc...
Probably not the best strategies since I ain't good myself but the point is, can you imagine the further chaos this would cause?
AeroWaffle2 votesBy "swap" do you mean swapping the positions? Like Flash being on "D" switched to Flash being on "F"? Or do you mean the ability to change which summoner spells you bring with you?
The former is fine, though would likely take a lot of programming to make it work for a very small QoL change that no one is really requesting.
The later, not so much. Enemy summoners are unknown to you until you hit the loading screen. By giving people the ability to swap after the match started you're allowing people to change their tactic based on what the opponent was starting with or what role the opponent revealed to be filling. Sometimes it's a good tactic to pick multiple champions that can fill multiple roles to confuse the enemy about who is going where. This would include trying to get the enemy to pick summoner's that are ill-suited for their matchup.
