In Mentals you can:
Combine different elements to discover new ones.
Fill up the wall of discovered elements.
Collect from your generators and trade the elemental gems that you have to find new combinations.
Sell different gems at different prices, to gather gold and buy new generators.
Complete quests to unlock generator slots and get gold and diamonds. You can use them to speed up finding all the possible combinations and elementals.
New elementals will be coming soon...