I'll take a stab at it:
All these off the top of my head. I expect many will lead to areas in need of development, hence the impetus for your post...
... ok, so I was gonna tackle it on a world as a whole basis, but then I re-read your post and decided that you are correct that a country by country approach is preferable. Basically we are writing a Gazetteer with these I guess?
As Frodia's founder and Champion I guess I'll start there. This, naturally, is just how I see Frodia in relation to GW. Most of this will probably be wrong, and I’ll have simply forgotten that we changed it. Strap in, let’s roll!
1. What is the deal with my cleric's religion?
This brings up the first sub point - the GW pantheon. I think this needs to be its own development project. Most of the gods will be aspects of the light and dark I guess? There should be significant overlap among the human population I'd guess? Many demi human races here too - they have racial deities?
Most spell casters in Frodia revere mana itself (the invisible mystic force that allows magic to function) and they draw their power from it for their ability. In D&D game terms, clerics "pray" to mana itself for their spells, while wizards memorize spells from books and sorcerers free-form manipulate the raw energy - the key is that all 3 are doing the same thing - conditioning their brains to allow for the manipulation of mana energy into magic spells. The exact method (rhythmic prayer, deep study and memorization or intuitive alignment) vary from class to class, and the types of magic they are able to manifest might be different, but the process is basically the same.
Some minor sects revere Frodal himself (leader of the rebellion against Garnia and the founder/first ruler of Frodia) as a god, though they, too, focus and harness mana similarly to all other casters.
2. Where can we go to buy standard equipment?
Just about anywhere. The Frodian economy is strong. Every settlement of hamlet size on up will have at least a general store that will stock most goods. Prices may vary by location. The cities will have thriving markets and traders of all sorts. The roads are generally safe and patrolled, and inter-city and international trade is common.
3. Where can we go to get platemail custom fitted for this monster I just befriended?
This one might be a bit trickier. I’m guessing the capital would have an armouror up for the task, but he probably is busy working for the King…maybe there’s something the King needs doing that could result in a boon?
4. Who is the mightiest wizard in the land?
I don’t know the naming customs of all these places yet (linguistics is Will’s bag) but I can tell you that he is the Headmaster of the Royal Academy of Magick, the largest university of spellcraft on Garnia (as far as humans know). Located in the capital city of Cothe.
5. Who is the greatest warrior in the land?
No names, and frankly, I’m not sure on this one. I’d expect the leader of the King’s vanguard to be pretty adept. General of the armies? It is just as likely to be a passing hero NPC (or PC) frankly. Most warriors don’t last long in battles with most wizards in these parts. The country of course fields infantry militia and the like in times of war, but the specially trained battle mage battalions do most of the heavy lifting…
6. Who is the richest person in the land?
The King. I’m not sure he’s called King, he probably has a more wizardy title, and he is always a Spellcaster (though his kingly administrative duties usually preclude him from the focus and clarity of mind to rival the top spell casters for raw power). Frodia is a feudal magocracy. Most noble houses are casters. The few noble houses that arose from non-spell casting lines are generally looked down upon slightly in noble society. There is a definite prejudice against those without magical affinity at all levels of society in Frodia. There are some rather wealthy merchants and smugglers along the coast though…
7. Where can we go to get some magical healing?
Swing a dead cat and you’re bound to hit someone…
8. Where can we get cures for the nastier bits?
Medium sized towns should be able to scare up enough power for most of the simpler stuff (poison, disease, curse) large cities or secretive powerful hermit casters for the more potent stuff. Death is a different issue. Resurrection is a highly controlled magic in the wake of the terror that was the Necromancer. Anyone seeking resurrection must apply for a resurrection permit, which is granted on a case-by-case basis. Non-sanctioned resurrection is a crime punishable by death and magical entombment which will preclude all revival methods, for both those resurrected, those performing the act, and those commissioning or abetting the act.
9. Is there a magic guild?
You better believe it. Frodia is a highly literate land ruled by a committee. You bet there’s red-tape. All spell casters are required to be members of a guild (there are many, not only by class, but by ethos, type of magic, etc…all are basically the same). Many guilds disagree on many things, leading to a certain chaos…one thing is certain though – keep SOME guild card handy if you cast a spell. Unlicensed magics can result in heavy fines and/or forced labor imprisonment.
10. Where can I find a sage/alchemist, etc.
Magic related trades will be found everywhere…typically settlement size correlates to ability level of expert. Other experts will be in bigger cities only.
Sure, mercs. Again, medium sized settlements and up will usually provide a recruiting ground for these folks.
12. Johnny Law:
As I mentioned, foreign spell casters need to register and join guilds. Unauthorized magic use is a crime in all of Frodia. Most folks wear swords – the land is patrolled and settled, but its still a fantasy game world – there’s some crazy shit out there!
13. taverns are commonplace.
14. Legendary critters?
I’m not Dming here so I don’t know.* You name it, there’s probably a place to find it…there is a fabled far off land of dinosaurs
*by which I mean this large overview has nothing like that. If I ran a game in Frodia I’d populate it like this, so just a question that doesn’t translate as well for our purposes…
15. Are there any wars brewing?
IIRC we are in a bit of a Pax Garnia period? I guess we should stir some shit up somewhere…is Garnia still fighting the Ming Liang?
16. How about gladiatorial arenas and whatnot?
Large cities for sure. Magical combat too.
17. Secret societies.
Probably. I guess this is a good development opportunity area…goes back to the guilds as well…
18. What is there to eat?
Typical fantasy world fare. Land is good, fertile and plentiful. Trade is abundant. Standard of living is fairly high.
19&20. Legendary lost treasures/ nearest dragon (or other rich monster) – see #14.