NFP was kind of weird to me. Because it was neither a big expansion as R&F or GS: no big mechanic introduced, but neither the small DLC as expansion Pack. It was kind of in between.
The NFP gave us more than new Civilizations and Game Modes. It introduced new mechanics like the 12 City-States, new resources (Honey, Maize), districts (Diplomatic Quarter, Preserve), Natural Wonders (Bermuda Triangle,Paititi, Fountain of Youth), World Wonders (Biosphère, Statue of Zeus, Torre de Belém, Etemenanki), Great People (strong one).
There were some small mechanic changes. For example, the Ethiopia pack changed the yields from City-States. Instead of being 2, 2 and 2 to the District, tier1 and tier2 building, it is now 1, 2 and 3 to tier1, tier2 and tier3 building. It isn't a small change.
The frontier between what is "small DLC" worthy and "big Expansion" worthy is up to you. I believe nobody will have the same answer. Yet, the way it was handled, I believe the NFP defeated the idea of "pick the one you one".
For example, you kind of need to buy them all if you want to build the Diplomatic Quarter for extra Envoys, counting on Chinguetti and Hunza to power up your trade routes, and use that faith on Naturalist that will get have National Parks around Preserve, counting on the Biosphère to power-up the Appeal of Marsh on the Wetlands map.
This is highly specific,I know.
The New Frontier Pass was a great idea to keep the players spellbound. It allowed the players to fully integrate the new Civilizations, Game Modes and mechanics before the next wave coming. It was more successful than the C&S Pack. Yet, in retrospective, I am more doubtful. The NFP had its own limitations (have to work on standard rules), which limited the creative process but not much: sure NFP isn't really balanced, but feels unique. But it left us with some frustrations: Barbarian Clans mode and Corporations and Monopolies mode are worthy to be base mechanic, but they were not well-thought enough to be a staple.
In short: NFP was a neat idea but left me with a bitter taste. Yet, I don't know how they could have changed the formula without ending into a new C&S pack that felt underwhelming.