Achievements and Daily Quests
Right now, this game is awful for new players. Most picked/desirable champions cost over 4800 IP and they get ~100 IP per win in addition to the first win of the day boost. But maybe they decide to stick with League for a little bit longer. The amount of time it takes to get 6300 IP is roughly 46 hours. That's probably about 15-20 days for most players to get an expensive champion. Also, that fun champion that they were playing for a week becomes unavailable to them for 2 months. This game also doesn't really have a reason to player more then to get the first win of the day. This game would be much more appealing if You got 300 IP for completing whatever task/ winning X amount of games with whatever role.
The Achievements could just reward players for reaching Mastery Level 5 with a certain role( could reward players with the old team builder icons) and giving people champion selectors every 10 times their summoner level goes up l up. 3150 at level 10, 4800 at level 20 and then 6300 at level 30. This actually wouldn't be an efficient tool of trying to instantly unlock a new champion because by the time you get to level 30 that's already about 10,00 IP
These are only a few examples but League hasn't really changed anything up for a while and right now the new player experience is pretty fucking shitty