I once tried to do something similar but found real-world survival a bit too complex to recreate the feel of well. I also studied californian ecosystems in college and might be pushing the realism a bit hard, and doing all of these would bog down game play :)
Also, here is a great thread I found helpful: /threads/rzs2q5qt?Wilderness-Survival-Adventure.
Real-world wilderness survival is hard because 1) without weapons, our food options are generally either low calorie or require a lot of time to catch (think Survivorman eating grubs and setting nooses for rabbits) and 2) even with weapons big game is rare. In Pathfinder, both these problems go right out the window. Kill just one edible enemy and your party is set at least for the rest of the day as long as they have time/resources to prepare it.
I don't know what sort of creatures you were planning on using, but I would think the majority would need to be inedible or have a high CR so it wouldn't be a free buffet. Deserts in general have mostly bugs and rodents, but not many big animals. Food is just too scarce. Abilities that can be used to detect might make hunting too easy as well. Undead and constructs might be good to use, but I don't know much about the Underdark.
As you mentioned both places having an oasis, that would be an ideal place for the players to set lures and traps as many other creatures would be using it for resources too. However, it would be quite dangerous to hang around since it will also be a prime hunting grounds for predators.
As for water, deserts are VERY DRY (and often blisteringly hot in the day). IRL water requirements per person in a desert is around 2 quarts of water/hour of hard work (fighting, traveling, hunting) and 1 quart/hour of rest in the cooler parts of the day, and this doesn't take into consideration things like food preparation, medical attention, or hygiene (per army guidelines). So if the party is doing stuff from sunrise to sundown, each person needs 24 quarts of water. That's a lot of water to have to find, and water is heavy. The smart decision for the group is to work at night and sleep in the day, which cuts water needs down to only 12 quarts per 12 hours. However, many predators use this same tactic in deserts, and will make night travel more dangerous.
Another idea is that the characters are probably either going to have water skins on them, or will want to try to improvise one out of a bladder or something. Depending on how far they have to travel away from the oasis, you might need to take portable water into account.
One aspect you didn't mention was environmental hardships. Heat stroke is a HUGE risk in hot deserts (over 100 degrees F) and nights are just as harsh (less than 20 degree F). They are windy no matter the time of day and make dehydration even worse. 100 F while in armor I imagine would be akin to sitting in a car with the windows up in summer.
The Underdark, I'm guessing, will be very damp since you are mentioning edible fungus. If the party is there for longer than a week, any metal, wood, or hide will start to decompose. They might have to roll for infections like foot rot unless they find a way to dry out every night.
Spells/abilities that give environmental resistance/disease resistance would largely negate the increased water needs, heat stroke, the cold, and the dampness. Know Direction would make navigation significantly easier too.
Sorry for the huge post! I hope I gave you some things to think about, and I'm very curious to know how you plan to balance the realism while keeping it fast-paced.
So far, all the GMs I have played with allowed us to choose our age/height/weight. In fact, the only time I have ever seen the age/height/weight tables used was for generating NPCs.
As for your Aasimar example, I once wanted to play a really short half-orc, and the GM allowed it as long as it was role-played out(both the advantages and disadvantages).
That's been my experience, anyway!
While I love Mysterious Stranger's idea of permanent magic mouth, I would feel a bit cheated as a player to get some really cool loot only to have it be turned useless or unhelpful.
I like the idea of offering three free permanency spells, but I would discuss it with the player first to make him feel like you aren't trying to punish him :)
Thank you for bringing this up! I completely forgot to plan for my player figuring it out early! It would be cool to reward my player for figuring it out while keeping the early reveal from derailing the quest.
Could you be more specific about why you would go another route entirely? Like I said, my only player is my husband, and I know he enjoys games with stuff like this.
I want to include a villain who is undercover as a good guy who goes with the party on a long quest (as part of a player's background arc), and I want to play up the betrayal factor as much as possible. Some ideas on creating emotional ties to an NPC and on balancing foreshadowing without making the twist obvious? Have you ever played in a game where a betrayal was especially memorial?
Some background info:
This is my first time as a GM. My spouse is the only other player (although I have a character too). I'm running a heavily modified Rise of the Runelords campaign. The arc is Resident Evil based- think horror mystery mansion with lots of puzzles and undead/aberrations.
I can think of a few ways of making the villain trustworthy using a few methods:
IDK I'm not really good at this XD