If a 4X game ever had 10 plus expansions, it wouldn't be Civ. Civ is too mechanically limited by historical realism
I don't agree to both assertions here.
Frist, civ isn't limit by historical realism. While it does create a lot of immersion (especially once you are used and accept the long standing immersion killers), most mechanics are very abstract and are only loosely built on history and far away from realism. If you want a game that cares about historical realism, Egypt: Old Kingdom is a good example.
Second, a civ game could easily have 10 expansions. For this to make sense, however, you need to stop thinking of expansions as 90s and 00s add-ons. Sadly, Firaxis has not made the step away from them either, so you are not to blame. What Firaxis does is add a wild mix of unrelated mechanics to the game in its expansions. These mechanics rarely tie to existent ones and are mostly superimpositions, creating more and more rather independent layers. I agree that having 10 expansions like R&F and GS doesn't make much sense. However, if Firaxis would focus its expansions on a certain aspect/mechanic and refining/redoing of current mechanics, it could be quite different (again, looking at the grand strategy games with 10+ expansions out there, this is what they are doing). Concentrating on a small number of highly related new aspects could also guarantee that they tie in neatly with existent mechanics and lead to more interesting decisions. The mechanics themselves need to be more complex or very handy if they are the focus of an expansion, they cannot hide any more behind others and a lot of new civs. This would, of course, mean somewhat smaller expansions, and even fans might skip one if they don't like the focus.
For civ VI, it could have been something like this: (note that I don't want to go into discussing the ideas per se, but just show what expansions could be)
1. Loyalty & Governors
2. World Congress, Favors, Emergencies & Grievances
3. Golden Ages, Timeline & Hall of Fame
4. Natural Disasters & [Random Events]
5. Future Era and Global Warming.
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6. Adding a first era before 4000BC that goes more or less Nomadic Mesolithic -> Shepherd Neolithic -> PPN -> current game start. Good candidate for an expansion some might want to skip.
7. Redoing Trade, Corporations, manufacture goods from bonus and trade goods. New Great Traders.
8. New systems for tribal civs.
9. Redoing the tech tree: you research core technologies/sciences that lead to different sets applications, a bit like in BE, just that the applications are not unlocked with amassing science, but by pop. No more/much less science from pop. Alternate set of buildings for the campus district. New Great Scientists.
10. Redoing CulV: new buildings for the Theater Square, reintroducing hotels, advertising campaigns, ideological works of art. New Great WAM.
11. Reformation mechanics for religion, new religious interactions (using inquisitors in your cities creates grievances and gives CB to civs with this religion for example). Possibility to become a secular civ in the later game stages.
12. Vassal states, Tributaries, betraying alliances, agenda rework, new policies.
13. Redoing the unit lines for ships and aircraft. Military buildings for the harbor, new buildings for the Aerodrome. New Great Admirals and Generals.
14. Introducing the Enlightenment Era.
15. Redoing fighting: units can stack up to 6 on a tile and fighting is tactically like in endless legend. Again something that quite a few people might not buy or disable after a few games.
15. Something I haven't thought of yet
Of course, none of these would have 8 new civs, but maybe 3 or 4 and an alternate leader. They could also include a wonder. The civs would not always need to fit the theme (looking at #6). Naturally, they would not be $40, but between $10 for smaller ones and $20 for the normal ones. Additionally, there could be Content packs: new maps (real and randomly generated), new city states, new resources, new wonders, new natural wonders.
So I don't think that a civ game with 10+ expansions is impossible or would change the game to be not civ anymore. It would have been quite easy to do if they planned it from the beginning and it would have led to a long life span with two expansions a year, creating a lot of constant and loyal customers - even if the high price tag for a complete edition might scare some. If they start to do small expansions after doing two big ones with R&F and GS, it would just create bad community feedback. And as civ history goes, an expansion that's adding to the late game (which GS is) has a high chance to be the last one anyway. Maybe we'll get a more contemporary DLC/expansion model for civvii or the potential civ vi engine offspring.
Age of Wonders is a great series. Since Heroes of Might and Magic has somewhat fallen into mediocrity, Age of Wonders continues to be a staunch (and wonderful) rival of Civ's in the turn-based game genre.
Really looking forward to the next Age of Wonders sci fi spinoff. In my view the gameplay in recent Age of Wonders installments is more engaging than that of Civ, but Civ's historical flavor is a more appealing theme to me (even if Firaxis doesn't always translate that theme as well as it could, they still did some great translations of historical theme, as with Saladin or Catherine de' Medici for example).
I believe that some people at Firaxis exhaled deep and relaxed after the announcement that the next AoW will be Sci-Fi. If AoW would turn into a history based game for one installment, it would be a serious competitor.