Vydaera

Professor Oak
Guybrush Ulysses Threepwood
Abed's Evil Doppelganger
Robot Steve Irwin from the Future
The goat from Goat Simulator
Seth McFarlane
 

Are you looking to build your own Avernum-styled classes and monsters in the Pathfinder system, or are you looking to play existing Pathfinder content in an Avernum-like setting?

Creating a lot of custom content might seem really neat at first, but often ends up being really limiting in scope versus the standard classes and such. I would recommend thinking long and hard about what really makes Avernum unique and just re-skinning a few things here and there to fit the flavor you are going for.

Some key points I would focus on:
The 3 unique races, Nephi and Sliths and Vahnati have their own cultures and attitudes

Most monsters differ slightly from standard D&D/Pathfinder, note the differences and play up the uniquenesses
The gaunt, pale, and for the most part, sickly appearance of the Avernites

The camraderie (spelling?) of Avernites, and how each individual contributes to the whole in unique and unconventional ways

How broken, roughshod, and jerry-rigged most things down here are and the contrast between Empire-built structures

Use optional rules for piecemeal armor and be liberal with lightly-enchanted parts

Mayors being major questgivers

Lots of secret doors, levers, and switches

Having Sages that identify items and will sell obscure knowledge

Bandits and Thugs are a thing, and they love to build castles and fortresses

Severely limit the learning of spells of 3rd or 4th level and above, make them feel like treasures to be found

Flesh out different religions, especially the Anama Cult
Option for Mages to have studied in the School of Magery or Tower of Magi

I guess give all characters an optional background in something unique to Avernum, like the Scimitar
*Mushroom Beer
*Don't ever let them catch a wandering herd of cave cows, even if they are resourceful and innovative enough to take down a fully-grown dragon

*Only let them have Swords and Spears, tell them bows are underpowered frequently

*Encourage them to play casters or caster hybrids

*Nobody can swim, boating is an option though

Take all entries marked with * with a grain of salt

I guess I kinda went overboard

I just love this series so much!