I've spent 700+ hours playing Civ5, and while it's been fun, the limitations of various aspects and dissatisfaction with the available mod options for invigorating the experience in the right ways have led me to think about trying out Civ6, since I've been hearing about how more intricate it is in many ways, and I just bought the anthology edition from Steam as it was on a hefty discount.
That said, I want to ask a few questions first before I actually start it out.
1. As a Civ5 player, what should I expect would be significantly different in the game mechanics when I play Civ6, be it new additions, changes to otherwise shared mechanics, or certain elements being removed altogether?
2. Any QOL mods that I should know about, especially ones that address shortcomings/bugs in the game that linger on even after (or even as an accidental byproduct of) all the official patches and DLCs?
3. Any leader/civ mods that are highly recommended? I'm especially interested in mods for the Arabians, Ottomans Germany and Russia, particularly if they can at least emulate the spirit of those civs' Civ5 uniques and leaders that they lack in Civ6. Civ5 Germany's Furor Teutonicus getting discarded for radically different abilities in the widely popular Vox Populi and Lekmod especially irks me; I wouldn't be opposed to, say, a mod for Civ6 Germany that has Bismarck as a leader option that focuses on "blood and iron" style diplomacy and governance, and Alaric I as another leader option that embodies the "Teutonic barbarian fury" aspect.
4. Is it possible to install mods as pseudo-DLCs to avoid interfering with achievements? That's something that has been really helpful in Civ5.
Welcome! Some of what is below has been touched on by others, but here's my take... fwiw, I have gotten all of the Steam achievements in both 5 & 6.
Point 1. To me, its a mixed bag in my mind as to where 6 succeeds and fails vs 5.
The Good:
1. I like the way Culture is handled. It's a definite improvement as Civ 5's culture was easy to smash, esp with certain civ and wonder combos. The Civics tree makes a lot of sense. Now a nation can be socially advanced and technologically backwards vs. another civ, or vice versa.
2. I like the new city layout with districts, and that you can't just have one (or maybe two or three) cities to build ALL the wonders in. (Though having a "production city" a "trade city" a "religious city" and a "culture city" is still a mostly valid strategy.)
3. I love that Governments are back again. IMO this was a failing in Civ V -- that you couldn't choose how to rule, as one had been able to since Civ 1.
4. The map concept is simple, yet awesome. Where you've explored, you know what is there but it's in grey. If you have sight into an area, it's in color so it pays to have bases and units in strategic areas. The overlays are useful for a change as you can now see where you can and cannot build, what the land values are by color code, etc.
5. The envoy system with city states is also a game changer, you can't just "buy the win anymore". That the city states have more flair in goods and diplomacy is also good.
6. Being able to choose (or buy) specific great people or pass to get the ones with attributes you want is cool as it allows you to tailor your nation according to its cultural advantages.
The Bad:
A. I
REALLY HATE what they did with roads. It's historically inaccurate, and it slows down the game needlessly, as it's pretty much impossible to link up even a small empire of 6 or 7 cities until well into the Industrial Age! Those Military Engineers are *expensive* and building roads is now basically pointless -- 3 MEs for a road between two cities 6 spaces apart?! No, thank you.
Worse, railroads are
also worthless which is also historically inaccurate and further slows down the game. In most games, I have operational airports before I have railroads between my cities.(!)
B. I do not like what they did with workers, it really stunts the game. I'm paying a gold per turn for that worker, why do they disappear after X uses? I could see if they raised the rate every age so they got more expensive and/or charged a fee to upgrade like the military units or something, but it just feels clunky and cheap as a mechanic.
C. Civ 6 is too easy to defeat economically. The AI is worse than Civ 5's for trading and will often give you wild values for goods, and you can often get 10 (gold per turn for 30 turns) for 150 or 200 gold or some other absurd ratio, depending on various factors. So you can literally "beggar thy neighbor" into poverty most of the time.
D. They over-did the district improvements. It defies logic that a city can't build both (say) sewers and a lighthouse at the same time, and then made it so there are so many little-but-important things to build. Sure, eventually you can get it all done, but it becomes an exercise in monotonous micro-management if you have more than a dozen or so cities, IMO. This of course is just a more detailed failing of every version of Civ -- that you can't build more than 1 thing a turn per city with production... no matter how much production you have.
E. The global warming is overdone/too touchy, and the advancements to reverse it come too late in many games. Yes, you can delay taking Combustion for a long time and not build Coal plants, but in the end... even your microscopic warming won't matter vs the AI's building coal plants just to burn coal. Though I do like that if you build enough "carbon capture" that you can set off an Ice Age.
F. Barbarians now flee... forever. If you start beating one and they can run they will do so. I have chased barbarians over 30 tiles. This is a mechanic from Civ 1 that I really didn't miss.
Point 2. There are a lot of cool mods out there, but IMO the one that matters most is "Extended Policy Cards". This should have been a part of the game itself, and not a mod. Just knowing what the effect of the cards are in your Civ makes the game MUCH better to play.
Point 3. The built-in scenarios probably have the flavor you're looking for. I really enjoyed the Australian (Outback Tycoon) and Poland (Jadwiga's Legacy) content in particular.
Point 4. I've not had any issues with a mod conflicting with an achievement, but OTOH I don't play with many (8).
Enjoy!