Just winging it so far, but as a starting point I'm sure we would agree it wouldnt be either appropriate or interesting for the choices to be identical, but I think the "Conquest/Trade/Decline" structure would work fine And, whatever they be, the crucial element is that there would now be *costs* associated with the choice. Cortes' Expedition, for example, was in effect a defiance of Crown Authority, but I don't have a clear idea how that would be represented in game. Stability, anarchy perhaps or, rather than poofing free units out of the ether, existing Old World units would form a portion of the New World force. I don't have anything interesting or original for the Trade option that would make sense. Granting resources comes to mind, but would these be implemented in the form of gifts from the in game civ or simply granted by script? Perhaps a nice lump sum of gold? Maybe a combination where the player gets resource gifts, only one city (or none) but a stronger garrison that is free upkeep, and Open Borders. In effect the player would be obligated to help defend with the added opportunity for slave capture but for no city or unit maintenance costs. Then for Option 3 maybe instead of Jack Squat a low-cost low-stakes transaction like idk map knowledge, slave trade, or heck how about a settler?
TLDR: its time to reconsider the You Won A Race, Here's a Free Army thingy.