I voted "pure historical civ", yet I also recently suggested elsewhere than an additional gamemode, where the gods and supernatural elements of religions are real and interact with one another, could have have been an interesting addition to the game. So this is obviously not a binary choice for me. But I agree with the OP that the main gamemode should try to be historical and feel plausible. Some people seem to like the lightheartedness of the series, but for me, the main draw of it, in addition to the strategy, has always been the pretense of watching an alternate history of the world unfold as I play the game. And this should in my opinion be the main focus of the game, when it comes to theme and atmosphere.
I think fantasy based scenarios and extra game modes are nice to have as well. There are usually not much negative aspects with adding these, only positive ones, but I do see one, which is part of the reason why these got criticized by some players in Civ 6. The problem or "problem" with these in Civ 6, was simply that some players felt that so many of the new gamemodes were fantasy based. And if they were unwilling to play them themselves, or at least not using them for most games, they felt "left out". This criticism could both be described as fair or unfair depending on your perspective, but it was probably the amount of fantasy-based game modes, rather than any single one that led to these complaints.
This aspect of Civ 6, along with some others, both in that game and Civ 5, does all factor together into making the recent entries of the series feel less serious and realistic than the earlier games. This a combination of several things:
The aforementioned game modes.
The higher amount of game mechanics in Civ 5 and onward, that feel abstract and game-like, vice versa those that feel like they could have a "simulation" rationale behind them. 1UPT, districts, certain culture mechanics are some that contribute to this.
The sterile and cartoon-like visual style in Civ 6 makes it feel less serious. This is by far the thing that is causing the most problems for my immersion.
Some other minor things, like the giant death robots that arrived with Civ 5.