Ordered from easiest to hardest to balance:

1.Cosmetic customization that is free(not cash-shop bound) and available through character creation initially.
Allowing fine control over character appearance and overall initial look.
Example:3D face tuning,decorative armor/clothes selection/customization, decorative pet selection.

2.Variety of graphical display of items(both on-avatar and inventory). Item modifiers should have subtle effects on color/glow/tint of item.

3.Many pets or allowing monsters to be tamed. Each pet would have its own mechanics and unique skills/abilities(c.f. Pokemon). Pet systems which engage the player at planning/controlling the stats/abilities/skillpoints use of pet.

4.Diverse character builds and viability of exotic builds.
Game should not revolve over few cookie-cutter builds, but some specialization builds which do one thing well at expense of everything else could be allowed.

5.Unique skills: mechanics of each skill should feel different from everything else in the game.
If several skills look and play the same they should be reworked.e.g. fire bolt<>ice bolt differ only in graphics and damage type->instead ice bolt should provide something different like AoE freeze from target or be able to freeze walls/objects to deal damage for nearby enemies.

6.Many unique modifiers and large amount of modifiers in general:each item would be drastically different, with few common points(item class). Many classes/forms/types of items, with each class/form/type having inherent stats/bonuses.