Riot Games has detailed their future plans for League of Legends in a lengthy blog post. During the post, they confirmed that two features that are highly requested by players won't be implemented.
Many players have been hoping for a Sandbox Mode where they can simply practice their mechanics without having to worry about ability cooldowns or enemies. Riot admits that it would be easier to practice, for example, using flash to get over walls if you didn't have to wait three minutes between attempts. However, they don't want players to feel like they have to do this in order to be competitive:
We want to make sure we’re clear: playing games of League of Legends should be the unequivocal best way for a player to improve. While there are very real skills one can develop in a hyperbolic time chamber, we never want that to be an expectation added onto an already high barrier to entry.
Learning through actual matches is definitely the more enjoyable option. Still, I'm not sure that the option of a Sandbox Mode would cause any harm. Giving World of Warcraft players the option to practice their skills on training dummies or in Proving Grounds doesn't hurt their enthusiasm for raiding. I personally think that giving players more tools for improvement and learning heightens their anticipation for actual gameplay.
Speaking of learning tools, Riot also revealed that they're not developing a replay system for the game. They admitted that it's their fault players expected replays, because they promised it at launch and even tested them publicly. However, they've since paused development of that feature for a few reasons:
We backed off replays because the technical demands (server loads, backward compatibility, network stability) were so high that we knew it would be hard to do them ‘right.’
Riot hasn't ruled out adding an in-game replay system. However, in the meantime, players will have to rely on third-party tools to record their games.
Part of the reason that Riot isn't developing replays is that they've prioritized other aspects of the game. Here are the features they're currently focused on: