Probably some generic tags with names taken by the port, in the manner of Victoria II ideology name and flag changes, would mean you wouldnt even need hundreds of unused tags for it.
Actually, if the converter was written in a clever manner, and exported the game as a scenario/mod you could actually have it create and modify tags/nations as needed. So if Burgundy survives into EUIII the converter could create a Burgundy nation in the mod folder for Victoria, hell it could even stretch/distort the flag from EUIII to match the V2 dimensions.
I don't think EU3 has enough demographic information in it to make a converter worthwhile.
EU3 only records the urban population, dominate culture and religion of a province. Without knowing the non-urban population, and what minority cultures and religions are in a province, you would get strange results in Vic2. For example, there would be no Jews, there would be no African slaves in the New World, there would no German communites in North America, etc.
It would only work would be if Paradox released another expansion for EU3, which would introduce alot more demographic information. Which would be very cool.
Nah, again you could probably write the converter in a manner so this wasn't so big a deal. You would use the vanilla Victoria 1836 population and demographics as a base. Then you could apply modifiers based off how long the province had been changed in the EUIII file.
So, for example:
If the Ottomans conquer Vienna in 1801 in your EUIII game and the culture and religion remain as they started then in V2 you'll use the vanilla V2 population stats and maybe just add a small amount of Turkish/Muslim aristocrats and beaurcrats.
If Vienna was conquered in 1720 and converted to Islam in 1750 then you would add a few more (small Ottoman) pops and set half the pops in the province to Islam.
If it were converted in 1600 you would progressively adjust more of the pops to be Islamic because it had been Islamic longer.
You could come up with some forumla, like: The base religion of the province is always the vanilla religion at the start of EUIII. If the province is later converted to another religion you can set half the pop to that new religion and then for every in-game year that it is owned by that religion convert another .1 to the religion in question.
So Vienna starts Catholic. It goes Protestant in 1520. It's now 50% Catholic 50% Protestant. It's owned by Protestants for 100 years. So it's now 60% Protestant and 40% Catholic. Then it's conquered by the Ottomans and converted to Islam in 1650 and owned by them for the rest of the game or 170 years. Right after conversion it becomes 50% Islamic, 30% Protestant, and 20% Catholic. After 170 years you get 67% Islamic, ~21% Protestant, ~12% Catholic.
You could make similar 'fixes' for culture and such. Then you've got balanced Vicky population levels adjusted by the history of your EUIII game.
Something like that might work.