I have some nice ideas for A5, I don't know whether or not any of them have been posted yet as I was trying to read through all the posts and my short attention span got the best of me...
First of all, on the surface a new race could be discovered. A race of anthropomorphic vulpines (or lupines if more people prefer wolves) could have remained well hidden in the denser forests of Valorim, remaining unknown to the other races in their secret forest cities. This could be part of a plot (maybe the Empire considers their natural stealth a threat and sends most of them to Avernum, or the Empire aspires to slash and burn their forests and sends them to Avernum when they resist). They could have bonuses to speed and lethal blow. I'm not sure what a fair experience penalty would be. It's time for a new race, since the original Exile/Avernum 2 have been added as playable. The vahnatai aren't entirely plausible, so why not continue the somewhat furry theme started by existance and playablility of Nephils and Sliths?
The return of crossbows as a heavier but more damaging alternative to bows would be nice, or perhaps a form of early firearm (flintlock or matchlock pistols, hand cannons, muskets, match/flintlock rifles and such).
DUAL WIELDING! DWing was the coolest aspect of the Exile series combat system and I'm sure I'm not the only Exile/Avernum fan who misses it.
A starting skill set that includes spear use, for the pole characters. Lancer or something like that since Soldier and Berserker both favor swords.
Grenade-like weapons: ceramic or glass containers with harmful substances inside. Incendiary fluid, poison gas, acid, antimagic energy pulse, caltrops, etc.
Another possible playable race that already exists in the Exile/Avernum series (though I can't seem to remember which game they were in) are the fungal humanoid Swamp Folk. According to the encyclopedia, they are intelligent and have their own society. They are territorial but not hostile, and capable of using tools with moderate dexterity so it's possible they would make good party members. I don't know what racial bonuses or experience penalties would be most fitting for Swamp Folk.