1. Level-free role playing system.
Character can earn points to improve abilities and learn skills or get talents for some achievements (e.g., being poisoned 30 times to get slight poison resistance), but his hp, damage and attack abilities (ratings) won't grow.

So you can go anyway from the start. And common skeleton in the beginning and in the end of the game will be rather equal enemies.

And legendary "Super Ultimate Divine God Slayer Broadsword", got at the beginning of the game won't be worse than common rusty kitchen knife found in the last town waste.

2. Accidental magical effects for low-skilled mages or for mage specialized on opposite mage schools (e.g., both fire and water)
Inspired by game "In Virtus Verbis" and wild mage from D&D

E.g., every mage can try to cast fireball (if skill learned), but mage with low intelligence (or fire magic skill) has chance to cast wrong spell (e.g. incinerate), or cast spell in wrong direction, or with some side effect.

Or mage with fire and water spells can cast fireball followed by rain

3. Character's appearance (and talents) change due to used skills. E.g., for continual skeletons summoning get charisma penalty and look like lich.

4. More strength and dexterity based skills