I think that your complaints are good but your suggestions are bad.
1 - Yes, Castile/Spanish mission tree needs to be revisited, Portugal almost always colonize the Caribbean Isles first and ignore Brazil (Brazil is normally colonized by England), blocking further missions for both countries, I am not certain about how fix it, maybe be a little ahistorical and give Tenerife to Castile in 1444 to increase his colonial range? Maybe one of the first Castile 's mission should become just have 1 explorer + explore the Caribbean coast and the bonus for completing it is a temporary buff in colonial range?
2- I disagree about limiting early colonization to Portugal and Castile, Portugal and Castile already have a geographical advantage in early colonization because they are the European countries less harmed by early colonization range, arbitrary restrictions are bad, advantages and disadvantages that organically emerges from game mechanics are good, in this case, the game mechanic responsible to give a edge in colonization for Iberian countries is the colonization range.
3- Yes, England AI should prioritize North America more and Dutch AI should prioritize colonization more, colonial rivalries between Portugal, Spanish, England, French and Dutch forming with more frequency would be a interesting gameplay in the mid game, if someday a new DLC focus again in the Americas, some new events chains or some new game mechanic incentivizing colonial wars with native americans allies (that results in border changes in colonies but not in border changes in Europe) would be interesting.
4- Ahistorical colonizers, if they are a rare occurrence, are good for gameplay and replayability, considering that they are a rare occurrence I don't see a need to change it.
5- You forget the main problem with the colonization system, colonial nations almost never reach their independence in late game against European AIs, I don't known what is exactly broken here, probably the mechanic of asking for independence support is broken (when decide if the AI will accept the request the game probably consider the distance between the European country and the colony asking for support, but it should instead consider the distance between both European rivals in Europe). Hold together a colonial empire in the age of revolutions should be harder, I suspect that we don't see much complaints about late game because almost no one play the late game (and almost no none play late game because late game have a tendency to be boring, after the early snowball there is no much fun in spend the rest of the game declaring wars against pushovers if you are not hunting achievements), a better revolution system and a better colonial independence system (both system maybe could be integrated in some way?) would make the late game more fun to play and would be more historical (and the little USA DLC and USA unique government forms would be finally usable), and yes, if colonies become more difficult to hold in revolution age, monetary gains of colonies in the mid game should be buffed.
1 - Yes, Castile/Spanish mission tree needs to be revisited, Portugal almost always colonize the Caribbean Isles first and ignore Brazil (Brazil is normally colonized by England), blocking further missions for both countries, I am not certain about how fix it, maybe be a little ahistorical and give Tenerife to Castile in 1444 to increase his colonial range? Maybe one of the first Castile 's mission should become just have 1 explorer + explore the Caribbean coast and the bonus for completing it is a temporary buff in colonial range?
2- I disagree about limiting early colonization to Portugal and Castile, Portugal and Castile already have a geographical advantage in early colonization because they are the European countries less harmed by early colonization range, arbitrary restrictions are bad, advantages and disadvantages that organically emerges from game mechanics are good, in this case, the game mechanic responsible to give a edge in colonization for Iberian countries is the colonization range.
3- Yes, England AI should prioritize North America more and Dutch AI should prioritize colonization more, colonial rivalries between Portugal, Spanish, England, French and Dutch forming with more frequency would be a interesting gameplay in the mid game, if someday a new DLC focus again in the Americas, some new events chains or some new game mechanic incentivizing colonial wars with native americans allies (that results in border changes in colonies but not in border changes in Europe) would be interesting.
4- Ahistorical colonizers, if they are a rare occurrence, are good for gameplay and replayability, considering that they are a rare occurrence I don't see a need to change it.
5- You forget the main problem with the colonization system, colonial nations almost never reach their independence in late game against European AIs, I don't known what is exactly broken here, probably the mechanic of asking for independence support is broken (when decide if the AI will accept the request the game probably consider the distance between the European country and the colony asking for support, but it should instead consider the distance between both European rivals in Europe). Hold together a colonial empire in the age of revolutions should be harder, I suspect that we don't see much complaints about late game because almost no one play the late game (and almost no none play late game because late game have a tendency to be boring, after the early snowball there is no much fun in spend the rest of the game declaring wars against pushovers if you are not hunting achievements), a better revolution system and a better colonial independence system (both system maybe could be integrated in some way?) would make the late game more fun to play and would be more historical (and the little USA DLC and USA unique government forms would be finally usable), and yes, if colonies become more difficult to hold in revolution age, monetary gains of colonies in the mid game should be buffed.
