Man this is really getting tiresome.
People flame so much about others performance in normal games, probably even worse than in my ranked games.
If you're not "allowed" to perform bad while trying to get better at a certain role or with a champion in normal game pvp, how then?
Coop can help to learn which skillcombination and skillorder get's the most out of the champion, but when it comes to actual fights or strategic trades, the bots are no replacement for players at all.
In teambuilder you have the option of playing the champion you want, but people are still going to be really pissed of when you make a bad play.
The Custom Games could prove to be useful, but has the opposite of an inviting vibe to it, because the amount of time it takes to find a group who wants to practise is like the one you need to find a pvp opponent in Dark Souls 1 below the 100 level mark. If you're really lucky, you may find someone, if not then you can end up waiting for hours until someone shows up.
And even if you find someone, it is not guarenteed, that the matchup is even.
Then we end up with the sandbox-mode discussion again, but nooo, Riot doesn't want us to get one...