If you want more champion diversity Rift will have to change fundamentally

Diversity is stifled by the design of the map itself and the constraints its objectives (primary, secondary, and tertiary) all impose.

People complain about ranged ADCs being absolutely required in every game. That's a combination of how the primary objectives are taken (towers/nexus) and how tertiary objectives are divided up (gold/xp).

People complain about 1-1-2-J being the only 'allowable' team composition. Again, that's due to how tertiary and secondary (jungle monsters) objectives are made available.

Want to really shake things up?

Remove ALL XP from minions and non-epic monsters and add an XP ticker like we have the gold ticker. Adjust XP values from champ kills and contested monsters to hit that ideal sweet-spot of snowballing that prevents games of Rift from stalemating. Have champions destroy towers via a ranged channel ala Dominion so that all champions are equally proficient at taking primary objectives.

By removing those two constraints the range of potential team compositions will get blown wide open.

Or?

We can keep doing what we've been doing since Season 1 and continue complaining about how the game never changes.

Riot could give it a whirl through the Rotating Queue to see what players think easily enough.