1)Improvement to the idol system: let us combine different idols together, creating a new idol with a combined shape (for example, attaching a 1×1 idol to a 2×2 one, resulting in an irregular form). It’s stats will be taken from the combined idols and enhanced randomly (or according to some rules) . This could also work between idols of different classes, tagging generally useful stats as “inheritable” (for example, chance to fear on hit from Acolyte idols).
2)An empty socket in the passive tree + jewel design: these jewels can be socketed in the empty slot and grant bonus stats, or give a passive node from another mastery tree of the same class, or even allocate one of its skills (with a small twist, like allowing Forge Guard to use Erasing Strike and converting its damage to fire). They could also affect some passives in the current mastery tree, changing them in certain ways.
3)Two new types of random rare encounters:
The first is a merchant who shows up in your base after completing an echo, from whom you can buy somethings good.
The second is having a chance, when entering an echo, to be teleported to a bonus level instead (something similar to the pony level in D3, fun and with a lot of loot).