Right now, Denmark basically wins the game if it gets about 10 cities. Its how the Runestone is designed, you eventually will just reach a critical mass where you explode. At that point of the game, the Runestone can almost win by itself (you only need to produce some science).
The Runestone would just be much better balanced if it wasn't constantly activating for every city, like if it was a national wonder, or if it only went to the nearest city. You could substantially increase the return if it was limited to just one city. That suggestion could work.
G's idea might leave Denmark still too strong. Basically its just production, but with a niche (but not weak) source of culture. Your culture output is going to vary a lot from game to game. Going just by theory, I still think its too strong. You still reach a point in the game where you have effectively infinite production, which is the main problem. Of all yields to have basically infinite of, production is really problematic because it removes so many decisions from the game.
With Greece, you get great culture output (and you can skip things like opera houses), but in other yields you are still playing a mostly normal game. Even with skipping culture in the mid-game, you still have a lot of yields to produce. You still want to grow, improve tiles, and make good build order decisions.
With Denmark, you can just forget all of that. You don't need to produce food or production. The Runestone will build everything for you. Why listen to that stupid supply cap? You could have -100% growth and production in all cities and be fine (it doesn't affect bonus yields, AKA the Runestone). If anyone wants to do a challenge photojournal, play Denmark and never use workers. You'll have to be clever to get strategic resources, but otherwise you'll be fine by the medieval era. You don't need your tiles, you don't need good city placement, you don't need WLTKD or golden ages or great people. All you need is science (which is mostly buildings and specialists) and a bit of gold for upgrades, and that's it. You don't need yields, you just need tiles to pillage. On a related note, never friend a city state. Pillage instead. A pillage every two turns will yield dramatically more yields than any alliance will.