[@SpookySquid]

1: Regarding setting and nations:
I think it would be best to do this in the 'Not so distant near future' so we have a frame of reference about history while allowing for customisation even if it means outrageous impossible things. Return of the soviet union! Federalized EU! United Commonwealth! And that way create a few 'Super nations' that can be relevant other than the usual USA, Russia, China actual trio.

2: Regarding the fantasy world:
Completely freeform in my opinion, lets go nuts, any race, magic, yay. Obviously would be wiped out of the map with a confrontation with any of the tech worlds in a conventional fight, but has the ability to go vietnam/afghanistan/iraq x1000 in the aftermath.

3: The portal: I'm thinking the nation hosting the portal should be the weakest politically and be forced to play a very tense diplomatic game. Alternatively: Everyone can open portals or there are many of them, conveniently one or two per nation, etc.

4: Nukes: I think nukes should exist and everyone in the modern world could have or use them if they so wish, and be able to use the 'If you do this, I'll nuke you' bluff/menace as much as they want. In fact, I could see this continue post-nuke when the fantasy world resources become many times more important.