Two suggestions to add replayability to the game
If I'm gonna criticise Plarium for the Iron Twins, may as well try to see if I can do better, right?
Realm of Chaos
My inspiration for this game mode comes from an old SNES game I played waaay back in the day called Lufia 2. That game featured a dungeon called the Ancient Cave. When you enter the dungeon, all of your characters are temporarily reset to level 1, and you temporarily lose all your items. Within the dungeon, you play the game as normal, you fight enemies, you get items, and you try to get as far down as you can. The gimmick is that there are certain special items you can collect within there, which you can take out with you after.
How I envision the game mode working for this game is similar. You start a "run" of the dungeon, and you can only have one run active at a time. Within the dungeon, you can use any champ you own. Rather than having them start at level 1, I figure it best to just keep them at whatever level they are outside, along with any masteries and books they've had applied to them. However, no items from outside the dungeon can be brought in. All champions will have a special second page of items, which will represent the ones currently equipped for the dungeon. That page will automatically become emptied (for free) when you start a run.
The dungeon itself will operate similarly to how the existing ones do. There'd be 25 stages of increasing difficulty, and each stage will feature two waves and then the boss. Each time you start the dungeon, the boss you get will be randomized (though it'll be the same for each run within the dungeon until you choose to end it). The dungeon bosses would be new, and would feature their own unique item sets. The items you earn in the dungeon will show up in your inventory as normal, and can be upgraded as normal, however, they can only be equipped within the dungeon (until you end it). You can end the dungeon at any time. When you do so, you keep any items you earned within it, and they become usable by your normal champs.
Seasonal Gameplay
My inspiration on this one would be Diablo 3. The seasonal gameplay of that game was by far the biggest replayability mechanism of the game. Prior to seasons, I played the game briefly, but never really got heavily invested. Seasons changed that entirely. They operated as a massive leveler of the playing field. Everyone started at the same point, and your ability to succeed wasn't impeded by how much time others had previously spent playing the game.
As with my previous suggestion, I'm going to try to keep this one simple. Each season would last a fixed amount of time - three months I think makes sense. After the season starts, any player can join at any point until it ends. The gameplay itself would be identical to the current game - you start, you pick one of four starter champs, and you grind/farm away.
The gimmick here is that each season would feature wildcards. For example,
- All healing effects are doubled
- All damage is halved
- Crowd Control effects have a hard cap of 75% chance of success
- TM Control effects are halved
- Item Set Bonuses are doubled
etc... I envision several wildcards being actived each season, and I'd hope that each season they'd be able to introduce unique wildcards to complement the standard ones.
The incentive to participate would be that each season would provide unique rewards. The easy way to implement this would be to have a special tournament and event that would be active throughout the season, and which would feature milestone rewards. They'd have your typical rewards, but also forge materials for season-specific item sets. They'd also have fragments for season-specific champions.