Given a late game with a large eco and good trade, you want a civ with expensive units that give a lot of strength for their pop space. Think mainly of civs with elephants, paladins and siege, or good unique units. Civs that come to mind are Persians, Franks, Khmer, Burmese, Lithuanians, Cumans, Spanish. Civs like Teutons, Turks and Saracens are also nice late game with unlimited gold, but are considered less strong in general and harder to get to the late game.
Trade means a team game and then the combination of civilizations matters. The first civ should be preferrably Spanish for the trade bonus, which means easier to get trade rolling and less trade carts required so more pop space for expensive units. Spanish have paladins and cannon units that are efficient use of pop space, and halbs when needed. Of course this is not a must to have Spanish to make a strong late game combination, but let’s just explore this option for now to make it easier.
With two civs in a team you would definitely want good archers, and if possible some siege/elephants/eagles to complement Spanish. Vietnamese sounds great because you then have rattans and elephants; or maybe Aztecs or Mayans for eagles, arbs/plumes and siege. Britons with champion and arbs/longbow or Goths for infantry spam sounds nice but in really late game their units are a bit cheap and not very pop space efficient, they would be better to win earlier in imperial age (also a bit true for Mayans).
With 3 civs the number of combinations possible is already getting very large, even with Spanish fixed. What makes a combo good I think is that you have as many of the following things as possible: one extra mobile civ; elephants, camels; archers; siege; eagles. Next to Spanish you could pick for example Khmer and Incas (lack mobility and camels), or Mongols and Vietnamese (lack eagles)
With 4 civs you could maybe have everything from the above combined and/or good infantry and cavalry archers and maybe some awesome unique units or an extra paladin civ. For example:
Spanish, Vietnamese, Mongols, Aztecs
Spanish, Khmer, Saracens, Incas
If you drop Spanish as constraint, you can make other combinations (don’t forget to include paladins and cannon units on the civ picks then). For example:
Cumans, Khmer, Indians, Aztecs
Franks, Mongols, Burmese, Aztecs
Of course “it depends”, on the map type, on whether you can survive to the late game with the chosen civs etc.