Russia colonised Alaska....it was nothing on the scale of colonisation on the East coast, and I suspect they only managed to 'colonise' so much because there wasn't any serious competition in the sphere. It could be an interesting aspect - but one that would also have to factor in that to get materials to/from Alaska, they had to travel across the expanse of Asia - much of which had only recently been colonised itself, and that for each winter Russia had no effective docks.
The Courland reference relates to Tobago in the Windies. Sweden had a colony that was swallowed up by the Dutch, which later was swallowed up by England. Personally I think Sweden, Courland and Denmark are to some extent represented by Holland.
I could be wrong, but at game start of colonisation I don't think Belgium actually existed.
As far as I'm aware the Ottomans didn't colonise in the New World. I guess their inclusion would be a what if? scenario....but the you could argue the case for China, plus a hoste of other European micro states such as Genoa and Venice which were associated with the seas.
Personally, I think the only really genuine alternative to England, France, Spain and Holland would be the inclusion of Portugal....but whether you could give them a niche to differentiate them from the others (England - high immigration, France - good integration with the Indians, Dutch - traders, Spanish - military and missionary) is another matter
