First off, Riot actually stated their reasons for not having a Sandbox mode. To paraphrase; they might explore more possibilities for you to try what you buy, but they're not adding a Sandbox mode because no matter how many times you practice your neat little tricks in it, you're still gonna suck when you try to pull it off in AI/PVP matches. League of Legends isn't quite like martial arts training - muscle memory won't help you. Even moreso, the mechanics of item builds are simple; buy what you value. If you value CDR, you're likely going to sacrifice the virtues of heavier damage or overall tankiness in exchange for being able to reel out abilities quickly. If you value damage, you're probably gonna come out into lane as a glass cannon. As for Champions, they each have their specific skills, and some people may have difficulty playing some champions. Warwick is meant to be a self-sustaining harassing brawler who ravages enemies and tears into them with his heightened senses. Fiora, I'll admit, has a trouble spot. She can't quite work things out if she issues her Grand Challenge on a mobile opponent. And, if it's your idea that you recall /immediately/ after killing an opponent with Grand Challenge...I give my condolences to the teammates you've dragged under.
Moving on to more complicated matters...Heroes of the Storm, as a point of reference. There's a lot of stuff to talk about here, so I'll get straight into it. Heroes of the Storm and League of Legends are two games of a similar type, but different work. In terms of AI, HotS is less organized than LoL, in that the AI Champions cannot be easily guessed as to which lane they're going. There's no method to it, you just have to hope you're lucky with your lane opponent. In LoL, AI matches are relatively predictable from the champion pool. HotS has an arguably better Champion set-up than LoL, having no item-buy reliance and starting out with all three basic abilities out of the gate, but at the same time, that reduces the necessity for adaptability. You don't have to purchase items to defend against specific damage types, you just have to hope you've got the modicums of healing available, whether it's Health Globes, Active Traits like First Aid (Or Laws of Hope in Johanna's case), Natural Health Regen (Regeneration Master, Reanimation for Leoric), other such passives (Butcher's Victuals), or active spells (Kharazim's Breath of Heaven, Rehgar's Chain Heal). Also, there is some certain unfairness in Heroes of the Storm in comparison to League of Legends, but also a little more balance. What I mean by this is; in terms of "unfairness", I mean specifically turrets. In HotS, Turrets do not target Champions who have hit you or another allied Champion. They only target Champions of there are no Minions available to hit. The balance comes more in Hero Strength. Note above, I mentioned the lack of Build Reliability. This means that you can't purchase to leverage yourself above enemy heroes. Which also means...If you fight one-versus-two, you'll lose. Whereas in League of Legends, if you have a proper build, you could annihilate an enemy team 1v5 if you wished. Although, that brings me back to the point mentioned at the beginning of the first paragraph; practice doesn't make perfect. Rehearsing your item builds and skills is not going to help you in a real match. There are a thousand and one different factors that come into play in a real match, that it's idiotic to demand a sandbox mode if it's not going to help. Bots have specific AI which can somewhat be worked around in most cases, but actual players don't. They have their own skillsets, their own strengths and weaknesses, and you aren't going to get the same guy over and over again.
While Heroes of the Storm does do Champion Mastery in a (in my opinion) better way than League of Legends, using HotS as a reference point to demand certain things in LoL is stupid. Blizzard will do their MOBA their way, and Riot will do League of Legends their way. Both games are fun and interesting in their own right, and I don't think anyone should complain that one doesn't have the traits of the other.
