@carlton_bach
Most interresting discussion. But Im afraid your solution requires a POD a little too early. I could be wrong ofcourse...
I'm not sure. The emergence of the plantation slavery system was a phenomenon of the 15th century, immediately following the establishment of the Atlantic colonies as sugar-growing ventures, and it came to be due to a scarcity of willing colonists, a growth market for sugar as a cherished commodity, and a supply of slaves at reasonable prices. If you dry up the supply at that point in time, you could force the hand of the locatores to find alternative sources of labour.
Regarding Iberians I see a Papal bull as a solution, but OTL saw the Pope trying but beeing unsuccessfull. Could work ATL with stronger commitment tough.
Bear in mind the popes also forbade bullfighting. How well did *that* work out?
If you had a strong current in the church (not just a pope) opposed to slavery on principle, that could do the trick, but IMO getting that requires a POD well before the fourteenth century. The medieval church was perfectly reconciled to the existence of slavery.
Does anyone have any ideas how to avoid the practis in protestant/reformed colonies?
One thing I could think of would be more religous based colonies and less merhchant based efforts.
The Protestant colonies adopted the slave system because they had seen it work. The pioneering effort was made in the 15th century by Spanish and Portuguese, everyone else just copied a winning design. Of course they *could* come up with it on their own, but there's no reasopn why they should, especially if there is an alternative model to copy.
And how would a North America develope without slavery? Indentured servants are ok. Would it develop much the same, but slower and less profitable?
I believe the development could be quite radically different. For one thing, the emphasis would shift. For well over a century, when the governments of Europe talked 'America', they meant the sugar islands. Certainly Massachussetts Bay and Quebec were all very nice, and Virginia was a genuinely profitable place (though, like Potosi, there could be but one Virginia), but if you were looking to make serious money in the Americas what you had to do was grab a Caribbean island, import some blacks and grow sugarcane. If that capital went into European settlement (of serfs or tenants/servants) instead, that would divert manpower from other settlement colonies and reduce the overall population especially of continental North America. If it didn't come into being at all, that capital will have to go elsewhere. Where it will go is anyone's guess, but the slave-sugar-trade was one of the money machines that fuelled much of Europe's prosperity in the 17th and 18th century. Finding something similar will not be easy.