Well it really depends on what they find, a good stat will be a roman army intended to conquer and than settle quite a big targhet, let's say Britain.
Let's suppose the invasion force in 43, led by Aulus Plautius, that we know was delayed by a mutiny of the troops, instead of being eventually persuaded by an imperial freedman to overcome their fear of crossing the Ocean and campaigning beyond the limits of the known world, where Lead in rebellion by that same charismatic man , to the very edge of the world, and effectively landed in Cuba...
The Conquistadores will likeley to find a pletora of successor states in the Caraibes, PAGAN romans...
This will lead to some intresting twists in the area as it eliminates some great waekneses the new world had in fighting of the Conquerors:
1) no resistence to european diseases
2) no centralized states in reciprocal competition
3) no domesticable animals except for dogs (assuming roman ones survied the trip)
4) no "high return" cereals except for mais, no large urban populations
5) no metallurgy (given the nature of the romans involved this is the tech less likely to disappear early)
6) no navy