Custom Games have been pretty much ignored by everyone, including Riot. It makes perfect sense why considering there's pretty much no reason to use them anymore. Outside of 1v1s, It's basically just a worse version of matchmaking unless you can find 6-10 other people to play with. Honestly, It's not even deserving of the title Custom Game. Beyond draft and map, there is **no** customization.
Lets fix that.
#Method 0 - Still not really custom
Just let us pick from existing RGMs. Its better than nothing.
#Method 1 - Buffs n' Sliders
This would literally just be a buff that's applied to certain players, team-wide, or globally. Let the player modify all of the stats that items already effect, and boom, custom game. It sounds kinda lame, but the most famous game mode, URF, is just a global buff. Riot already has the tools to do this, all it needs is a UI with sliders for a player to create a custom buff.
#Method 2 - Overwatch Style
The name pretty much says it all, but if you don't know how Overwatch's custom games work, here's a rundown. In addition to globally applied buffs, every character has their own unique set of sliders that can change speed, damage, healing, cooldown, etc. of every ability and gun the character has. This method would be MUCH harder to implement than just buffs considering that there are 140+ characters each with their own set of 5+ abilities (including passive).
For further customization, implement these modifications to **ALL** Items, runes, and summoner spells, as well as the ability to disable them.
#Method 2.5 - Simple Non-Player Changes
On top of one of the previous methods of customization, let us alter entities on the map. Meaning enabling/disabling turrets, jungle monsters, minions, etc (already in practice tool), as well as changing the behavior of those entities. Let us change Jungle monsters from how their normal function to an aggressive mode in which they will go after anything that wanders too close, or let them roam from their nests and patrol the jungles, or mix the two and have patrols of aggressive monsters that can and will attack on sight. Let us edit spawn timing, spawn location, teams, and types of minions. Let us turn turrets into neutral structures that defend and block off certain areas from both sides. Let us add Nexus fights from NB if we want games to not go for so long.
#Method 2.5.5 - Items
Let us create items using already existing items and effects. For instance, an AD/Armor item with the Gunblade active, but instead of slowing it disarms. Or Boots that allow the player to use Talon E on an ammo system. Also, let us start players with certain items and lock those items permanently.
#Method 3 - DotA style
Simply put, let us make our own fully custom creations from scratch. A TON has been done with DotA modding. They've made new maps, new AI, new objectives and even new CHARACTERS. I'm not going to pretend like I know how either the client side or developer side of this works, but this would obviously be the most preferred method.
Let us save and share our own creations with the world, as well as like/favorite/star these custom games to add a social aspect to custom games.
Make presets of existing RGMs for us to use and modify. (Hexakill, OfA, OfA: Mirror Match, Urf, Blood Moon, etc.)
Let us use the Star Guardian, Odyssey, Dominion, Nexus Seige, Nexus Blitz and Overcharge maps. Especially Dominion. Please let us play hide n seek again.
Considering Riots current track record with custom games, I don't expect a single one of these ideas to come true. However, this would MASSIVELY increase player retention. So many people are frustrated with current league because it's too fast, too competitive, unbalanced, etc. Allowing those people to create/participate in *actually* custom games can relieve a lot of stress, or let them experience the gameplay they miss. Silly non-competitive game modes are GOOD for the player base. People need ways to chill out and untilt, or else they might lose all the fun in playing and move to something else. If the Hide n Seek community could exist for such a long time with less then bare-bones tools, imagine how long new communities could thrive with a full toolbox.