[B0rsuk]

I think I have an idea how to make nationals scalable without increasing micromanagement.
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National troops would get predefined bonuses for each additional fortification you control. Makes sense, as you need them to produce national anyway.
Flavor-wise it would be something along the line "The garrison of Blahuageuawraarhsaark made some improvements to heavy crossbow design and we can now use it nation-wide" or " new forging techniques, or news of national hero/propaganda (inspiration;see soviet Russia for reference), source of better ore...

To stop Watch Tower abuse, it would be NumberOfForts*FortAdminValue (no I have no idea of Dominions variables).
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Edit

For those of you who think that "you can just start producing nationals for free - therefore they should suck"
- nationals require forts (say 300-450 gold, 3-5 turns to build, and a commander)
- Process of building a fort can be interrupted.
- forts need adjacent provinces to work properly
- resources, unlike gems, don't carry over to next month (seems like, doesn't it ?)
- there's a hard limit on how much you can recruit in one turn. On the other hand, you can burn LOTS of gems in a single turns.
- you can get gems from another player, not true with resources. Technically you can get away without magic sites for some time.
- you can keep all summoners in one spot, forts force you to expand.
- many national units require temples (usually 200 and a priest to build it) or labs (200 and a mage). Combined with 450 fort, this means 850 gold, multiple turns of building and commanders.
- I'd say population killing spells are quite effective. Growth scale is slow. Your ability to buy conventional troops is affected by cold, heat, unrest, productivity, order..