I always enjoy achievements that encourage players to play in different ways and explore parts of the game they may overlook when they get locked into certain strategies. Here are some ideas for achievements that do that:

Leveling up different types of equipment:
Level up a (type of weapon) to +6 ; Types are swords, axes, reach, fast.
Level up a ring to +6.
Level up thrown weapon to +3.
Level up a wand to +6.
Level up wizard staff to +6.
Level up an artifact to +10.

Using alchemy:
Transmute a (weapon, wand, potion, scroll) with the scroll of transmutation.
Use alchemy pot to create: exotic potion, exotic scroll, etc (one of each category on the alchemy page).
Use liquid metal to repair a thrown weapon.
Upgrade a wand with resin.
disenchant a scroll.
make and use the alchemy recipes with the costly items (potion of mastery, scroll of enchantment, potion of might, magical infusion).

Using inventory items:
Kill an enemy with a runestone.
Kill an enemy with a thrown weapon.
Kill an enemy with a bomb.
Kill an enemy with a wand.

Fighting in different ways:
Surprise attack the same enemy 3 times in a row.
Kill an enemy by knocking them into a pit.
Cause an enemy to kill another enemy.
Kill an enemy with a dart while the crossbow is equipped.
Kill an enemy with an armor glyph / weapon enchantment.

Exploring customization of equipment:
Augment armor for evasion/defense, then augment the same armor to the other evasion/defense.
Inscribe a glyph in an armor that already has another glyph inscribed.
Craft a scroll of enchantment and use it on a weapon.
Curse infuse a weapon.

Taking the road less traveled
Give the rat king what he wants.
Use the wand-maker quest item for something else.
use a scroll of upgrade on a cursed item unidentified item.

gotta cut myself off, went a little overboard. Thanks for your work on such a great game!