The frontier pass is a great package of civilizations, modes and maps that really adds a fun layer to any Civ VI game, and the model of delivery the contents was great too. Too bad it was a one time deal. Now it's 2 years later, let's take a look at the contents:
Civs and Leaders:
Mayans returned to the franchise focused on having a small core empire that rewarded city and district planification around the capital. The added layer of complexity make it an Civ more difficult to play and master, but very rewarding if placing tiles make you brain tickle.
Gran Colombia was a surprising new addition to the roster. Their 1 movement bonus is something that low key can make you explode mid game, as the number of things you can do are more than anyone else. Combine it with other movement bonus and you'll circundante the earth back to your mom's wom in no time.
My playthough with Ethiopians was exceptional. Their bonus allow you to keep on with other civs tech and civics by just adding faith stuff to your empire. Things can snowball really hard.
Byzantium imho is kinda lame. Yeah the make your knights basically a siege cannon, but requires to much plan, execution and luck that is not worthy, and after mid game, they are just bozos with lame religious gameplay. Not much on the brain tickles there.
Gaelics on the other side of the coin are just monsters. The early industrial district just breaks the game. You get so much done by having that thing early that production becomes trivial, you just add thing to the queue. Their downside being the weird district placements and bonuses is the only thing that troubles me, as it screws with all your pass knowledge of the game.
Babylon should be a banned civ. In the hands of someone that knows what they are doing they will be hammering you capital with cannons by the time you discover writing. Other than than I founded that I was doing just little things all the time instead of focusing on the big picture game, so I'm conflicted. Lots of brain tickles with every Eureka tho.
Sadly, I've never play Kublai khan. So I can say much other than the eco bonuses seem nice and synergistic with both Civs he's leader of.
Vietnam is one of more fun civs to play. Very easy to defend your territory, as your units will shred any invasor force. Also their restrictive district placements can also tickle monkey brain when the right resources stack with bonuses.
And finally, the best for the last, the Portuguese. A glorious return for a Civ that in past games of the series never shines as bright. My god, those water comercial routes are just bonkers. You will stack on stacks of gold. Production will worth nothing, as you'll buy units and building as your heart desires. Very satisfying Civ and best of the pack.
Modes.
The second best thing the frontier pass adds is the freaking game modes. Some can become core to your game session. Others can fall behind and be an occasional thing, but they complement the game so well that is hard to pass on them.
Apocalypse will make you just suffer. It's very fun and novel but extremely tedious in more serious games. It can pin down the IA so much that take the fun of having them in the game and the asteroids after a threshold of pollution is... PainFun.
Secret societies has been active in my games ever since. They can complement you Civ playstyle, but choose the wrong society to be part of and you'll thrown away those governor titles.
Dramatic ages change how era function dramatically (zing!).
Bye bye boring normal ages and prepare to lose half your empire in every dark age. This mode changes the core game too much for my taste, but I still think it has some grace and fun in to it.
Hero's and legends adds solid gameplay with new type of heroics units with special abilities. I usually play with this mode on just because the heroes are so fun and I still have to play with all of them. They can turn the tide of combats and troll opponents in novel ways. Recommended.
Corporation is the only feature I fell should be in the base game because it fit so well in the gameplay. Always have it on, bonuses are solid but not game breaking, everyone can enjoy them and Is very flexible to every game style.
Finally, zombie mode is just weird. It can be fun because of the snowball effect of zombie spawning. But also, it gets old quickly as they never stop spamming. And will do in any corner of empty tile. So, fun experiment but needs work.
And to finish my review, the extra stuff.
Natural wonders are a nice addition but very circumstantial as is never guaranteed you'll get to play with them, different situation as with world wonders, as you can actively seek them. Having more options is just nice, but is not the meat of the pass.
Same as great people. I have no idea who is new. But hey... More guys to my empire.
A real game changer: diplomatic quarter. Very nice district with buildings that add very interesting bonuses to boost your strategy.
Same thing with the preserve. Pivotal to some nice natural wonder sick yeilds.
And, to finish, the maps scripts are a solid extra addition. Their features are very characteristics, if anything way balanced and somewhat generic.
Conclusion: I believe this is a solid DLC for Civ. Many cool additions, expansive strategy and lots of brain tickles meet. The price is steep, but it feels like a modular expansion and you get to choose what to parts to engage. Many civs are top tier and I think it changes the game essentially. Remember that many features were released as free updates when each part of the frontier pass dropped, so plenty of things are in the base game that came at the time with the pass.
My only let down was that they never did it again.