-A. A game EXACTLY like MP1. All the broken mechanics included. Be peaceful, be mean, show restraint or zerg across Asia like Genghis Khan. Destroy opponents with a few carefully-executed espionage missions. Major NPCs go to lunch on anyone who’s small around them, or even human players.
-B. A game that is balanced and removes the combat elements in favor of economic and diplomatic elements, what MPV wishes to attain.
I would go with B. Take MP1 and just fix the clearly broken parts of the ruleset.
Like the combat system.
Especially the combat system.
-Zerging capacity for invaders (leading to the rise of superpowers like Xinjiang and later Scotireland)
Bring back the IOTVI-era XP system for expansion. Using armies for expansion was cool until everyone realized that it leads to steamrolling.
-Capability to nuke everyone without consequence
That...wasn't fun. Who thought that was fun except one or two people?
-Unchecked espionage that led to insane results such as an entire treasury being lost
To be fair, espionage is imbalanced because only three people ever use it. Probably because so many of us came from CivIV.
-Clients that easily betrayed you at the drop of a hat
Hurr.
-Major NPCs, controlled by yours truly, stirring the pot all over the globe
Double hurr.
-Insane tensions between Korea and Hawai’I that sometimes had meta proportions to them
Triple hurr.
As is well known, I think "fairness" is a load of hooey which should be dispensed with as the idiotic conception it is. As such I would have absolutely no problem with a game exactly like MP1, although personally I would prefer it if it didn't have nukes (nukes made it too easy to counter warmongers, and it would be more engaging I think to have failure to balance warmongers have actual lasting consequences). Other than that one gripe, I wouldn't change anything in the event an MP1 redux was initiated.
C-c-combo breaker.
I would get rid of nukes, or simply make them tactical only weapons.
I vote for a third option. While a rehash of MP I seems like a fun idea, the problem with broken mechanics is that they are, well, broken. Note that I have not played MP I, but I know from experience that being Zerged is no fun if you have no chance to get your land back, same thing with being nuked into oblivion and there are better ways to stir the global pot than Major NPCs. Trust me, I know this from experience of MPR + MP II-IV.

And I think MPV is a great idea, but that should probably be left to MPV, as a world has already been established that a global UN wouldn't work with. In my opinion, we could get a combination of all the best worlds from following the original design philosophy of ATEN:
Zerging only works on players who didn't rationally build troops out the gate. So many players think that having no standing army the first two turns is a great idea that examples of zerging are subversions of the trope. Mind you, I've SEEN a player lose a zerg, get knocked down to two provinces, then promptly conquered all of Africa.
I also disliked major NPCs, but I understand the need. I loathe to fully dissolve a player's country myself.
I would also get rid of the UN entirely. Let players work it out.
-No nukes, or relatively ineffective nukes (think weaker than MP IV) as they just turn everything into derp.
Indeed. I can mark the changing of the guard in MPIV by the time nuclear weapons were revealed (again). It seemed like my entire strategy changed on a dime and ended up revolving around nuclear weapons.
Because in a game where one weapon, and one weapon alone, is sufficient to turn the tide of world affairs, everyone will jump on those weapons.
-Relatively easy-to-conquer NPCs that give a bonus for conquering them, sure give them a revolt penalty as well but not enough to make them unattractive to conquer.
No. NPCs should never grant some mystical bonus for being conquered. This goes back to "army expansion leads to runaways".
-Hard-to-conquer PCs, well unless there is a major power difference between the two nations or the conqueror has nommed on a few NPCs beforehand.
Again, no. A player's ability not to be conquered should be that, an ability. A skill. Not something the game sets up. Players and NPCs should, as much as possible, play by the same rules.
-Cartoony NPCs (as in quick to influence) are cool, but majors are a bit much.
No. Just go with the "everybody is a NPCGM" thing in RIOT.
-A semi-complicated economic system that is enough to please the builders and the mathheads, but not too complicated to scare off everyone else. Maybe a test run of MPV's economic system?
You can't really build upon the existing economic system without going past the point of no-return. MPIV was probably reaching a brick wall in that regard.
-Decent sized RP bonuses so a good RPing nation doesn't get eaten by Genericwarmongeristan off the bat.
RP bonuses shouldn't exist and, if they did, they should come with RP penalties.
But if I could pick, I would choose a more MP3 esq game.
I say if the majority of people want a new MPIII I say thats what you should give them. I wasn't a huge fan of MPV, but MPI seems just too crazy.
I liked MP3. Had its problems, yeah, but MP3 was kinda the standout of the series besides 1. But if I had to pick between a broken game and a working game, I would go with a working game and therefore B.