Re unlimited subs - there's a few interprations of this. If it remains, once subbed off can't be subbed on again, then it's essentially just an increase to however many number of subs are on the bench... 6 outfield subs for PL... Which isn't a huge change.
However this changes by each tournament rules, so I assume it could mean rolling subs, players being subbed off and on again multiple times in a match. This would be an ENORMOUS change and would lead to specialists - a player being subbed on just to take free kicks, or a goalkeeper just to save a penalty, or Fellaini being subbed on just to attack a corner. This would be awful IMO and would be damaging to the game.