Riot, your fear of a Sandbox Mode grinding are really about only one spell...
http://na.leagueoflegends.com/en/news/riot-games/announcements/riot-pls
Lets break down the reasons people want a sandbox mode before we dive further.
- To understand items and their interaction with other items/abilities
- To understand what active items with long cooldowns can and cannot do
- To understand certain Ultimates (Bard ult, what can/can't I use it on. Can someone cleanse out of it? QSS? If they were in Zhonyas before? etc.)
- To get better at using Summoner Spells
Playing games of League of Legends should be the unequivocal best way for a player to improve
Of the above reasons for wanting a Sandbox mode, which conflict with this notion? Surely understanding League of Legends's mechanics should not be something that can only come after playing hundreds of games. That only makes it harder for new players to join the game. Better documentation by Riot on what abilities and items can do would almost completely eliminate this understanding section, leaving only the "I wonder how champion X works with full build ABCXYZ", which isn't grinding.
If we remove the points that are about understanding the game, we're left with one point:
- To get better at using Summoners Spells.
There are only two summoner spells that would be considered for this.
- What can I cleanse? Suppress? Bard Ult? Zed Ult?
- What can I flash over? What can I do while flashing?
Cleanse is no different from our previous cases, it's a quest for information about how a game mechanic works. A better tooltip on Cleanse could remove the need for testing it all together.
#Flash is the sole reason for Riot not wanting to make a Sandbox mode. Specifically, Flash's interactions with other abilities is why Riot doesn't want to make a Sandbox Mode.
Flashing over walls? With a map of what you could possibly flash over, color coded for difficulty, most players would be content trying this in game. That's not the problem.
Flash+skillshot? That's a whole other can of worms, especially for skills where you start the ability, flash, then the skillshot fires from your new destination.
Lets look a little closer at this Flash+skillshot interaction. When do you test this? According to Riot, in any ordinary game mode, even Custom. In PvP modes, burning your flash to try and figure out a spell could leave you in a bad spot. Needless to say, testing it in Ranked is probably a bad idea. Testing in Normals requires a decent amount of time, and you'd probably have to be willing to spend your flash every time it came up to try and pull off the interaction. Enter angry teammates reporting you for trolling/intentional feeding/unskilled/whatever for burning your flash for no reason over and over. So you're left with Custom games... isolated custom games, which while slower, are even more grindy than a Sandbox Mode would be.
but when that benefit is weighed against the risk of Sandbox mode ‘grinding’ becoming an expectation, we just can’t accept the tradeoff
You want to remove the possibility of grinding in a Sandbox Mode, Riot? Force all skillshots to fire from where they're started.
Lets be honest, it's more likely that there would be 24/7 URF mode custom games than there would be serious grinding, much less mandatory grinding.