I agree with points 1 and 2. I believe the /allchat function should just be removed, as it really serves no purpose for the game. Like, I understand that games are meant to have some little trash-talking, however, I feel like that isn't what's been going on. Trash-talking is meant to be something to "taunt" your opponent (you know, just like that /taunt command does) into doing something they normally wouldn't have, be it getting themselves killed by going in with no health/mana, chasing you right to your team because they had 0 vision, etc. However, a percentage of the community does not understand this and they seem to believe trash-talking means insulting their irl lifestyle/choice(s), insulting their families and/or friends, and sometimes worse, suggesting suicide (and this part isn't even funny, it's just plain cruel and unnecessary).
The only issue with removing /allchat for this reason is the fact that a lot of players don't just trash-talk their opponents, but sometimes their teammates, as well. It's only natural that people lose their games "due to teammates", especially since the community is making this game seem like it's a "every man for himself" kind of deal. It's almost like no one realizes that this game is team-based. Trash-talking teammates does nothing but lower their morale, cause them to perform worse than they more than likely were anyway, and it causes miscommunication between allies. Not to mention the discomfort factor that comes with playing with a flaming teammate.
If you ask me, the chat function should be removed altogether. Replace it with a Ping-only system and then add more pings to it. Want Baron/Dragon, there should be a ping for it. Wanna target a specific lane, there should be a "bot/top/mid lane" ping for it.
Note: Most of this is just me complaining about my experiences. Also, most of this is geared towards SoloQ. As far as MultiQ goes, I'm clueless to how that works out.
Point 3 is a hit-and-miss, in my opinion, because I view someone saying "gg" as a sportsmanlike concept. Whether they are actually being polite with it or not, isn't my concern. I just respect a game outcome, regardless of whether it was or was not a victory, with a "gg". HOWEVER, yes, I do believe that players muted in-game should be muted post-game, as well. Not having them muted post-game almost defeats the purpose of muting them in-game. Personally, I feel that the game should tell people when they have been muted (in-game), so that they know to use pings from that point on. Issue there is, you'll have those players who lose their shit because they've been muted and then will be talking to the rest of your team (as well as, the enemy team) about how you just muted them because you're a "whiny little b****" or "you're just mad because you're trash" or some dumbass shit like that. Basically, it won't turn out as well as I would hope for.
Point 4 would work if the RP system did not exist. Issue with getting rid of it, though, would be Riot loses money because...well, all the champs would be "free". In all honesty, though, I feel they would still make a bunch of money off skins, wards, Summoner Icons alone, even if Champs were free. If you ask me, Riot should do what Smite does and make it to where you can't use a champ in Ranked Play until you've mastered them to some degree. For example, the Champion Mastery system should be required to hit a certain amount of Mastery points prior to a player being able to use said champ in Ranked Play. This will also force players to actually LEARN a handful (if not a bowl-ful) of champions prior to playing ranked. Not only would this ensure they have a better time in their games, it would also lessen the amount of toxicity by even the slightest margin, since players will know the player has at least some knowledge and skill on/with said champion.