I think North America and Europe are no good idea.
Too much airports need demand from other areas, so for example Los Angeles is nearly useless while JFK would be much better than normal.
Also 150 player for two continents? Too easy for an Alliance Challenge.
Why not making an Euro Challenge out of it?
Maybe with no basing at Top8 or Top10 international airports.
However, for me personally everything will ok, I'm sure I can deal with the situation, but Europe and North America sounds very imbalanced to me.