i think i can clarify that a little more for you. when i talk about megathreads i dont necessarily just mean stuff to give riot feedback, although that naturally likely would be the biggest part of it. i also mean simple discussion about different aspects of the game both in and out of game. when i say strategy im referring more or less to discussions about effective ways to play the game to give it a very broad definition. for example, there could be a megathread for new players to learn that no you in fact should not attack the enemy champion under their turret (i was legit like level 12 before someone told me thats what is causing me to get agro'd), a megathread for players who want to hop into the role of jungle/improve their play in that area, even one to talk about ways to improve your individual and teamworking skills to climb the ranks. but of course it doesnt all have to be that kind of thing, you could have ones about eSports and fantasy leagues, favorite off meta playstyles, the comprehensiveness and overall quality and setup of the client itself, ways to deal with toxic behavior. i will say that there are some things that may not be a good idea to have a megathread about just because of how broad the subject is or simply that its something better discussed in smaller settings.
i think (and this is just my opinion from my own memories) that part of the reason the old megathreads didnt do amazingly well is because they werent always topics people really cared enough about, and for me at least it was easy to forget that they were there. megathreads can also be daunting in size, but that could be solved (if you have the ability to do this) by more concise formatting like having the thread default to reddit's format which i know, some people dont like, but overall its more organized if you ask me. another possible idea that i like is doing smaller megathreads weekly or bi-weekly for the sake of keeping interest fresh and just revisit the really popular ones with a link to and summary of the previous one on the subject.
another thing id like to note is that having mass discussions like this can bring together a lot of idea power and problem solving skills and bring up ideas and solutions riot didnt even necessarily know they needed or at least could use to improve player experience. Riot may not know how to ban players from ever creating a new account without possible unhealthy drawbacks, but maybe PotatoSalad42 has an implementable idea. additionally, it would allow the players to feel more like their opinions and voices.
granted i dont know what all is and isnt possible and what goes into things like this and policies that have to be followed and whatnot, but i think that if this is something possible the potential benefits would far outweigh the potential drawbacks.