I've been playing these maps with Gods and Kings recently and I'm enjoying them more than ever. That said I've had to abandon the Giant Earth Map, as each update for the Mac version of the game seems to have made it run worse and worse.
Anyway, I've been playing the large europe map and I've a few possible improvements:
Firstly, I think that Songhai could do with a starting location; I know it may not be accurate but it would help to contest the african continent a bit more. Currently I play with raging barbarians, otherwise Carthage and Egypt can quickly dominate the game, as even with the relative disadvantage of the terrain they can become very powerful early on. A similar start could possibly be added for China, and possibly Persia as well, to further increase competition in some of the less populated areas of the map; in my particular case I've found that after a slow start Arabia has become a powerhouse in the later game after dominating the entire middle eastern area, and are well into the atomic era while I've only just managed to catch up to the modern era. More competition also benefits the use of Espionage, as with it being (stupidly) city-based it can take a very long time to find a good target when a rival civ has 40+ cities!
Secondly, some of the new civs could do with more favourable terrain for their bonuses. Specifically, the Celtic Faith bonus requires forest near the start, which I don't think the map has quite got, as the forests need to be immediately adjacent to a city to have any effect, and it doesn't currently seem to work if the forest contains a resource either (very annoyed by that, not the map's fault though). Also, Amsterdam is a bit tricky as the Polder is a really powerful tile improvement, but their start doesn't have much marshland, though that said the AI would probably clear it all even if they did. It's a really odd set of bonuses some of the new civs have; means the can get very little value out of them depending on start, and on random maps they're more likely to never benefit. It's a tricky one, as ideally Amsterdam would have somewhere marshy nearby that an AI player wouldn't normally settle, but with Polders they could later benefit from greatly, but I expect that that wouldn't be geographically correct, and the AI would like just settle around it and clear it anyway.