No one is saying megafauna, though, let alone predatory megafauna. Assuming that vanilla DF as we know it now is medium fantasy, I am perfectly fine with low fantasy worlds having zero animal life in the caverns because all the plant life is based on heat from the magma sea and thus insufficient to support animal life, and I'm 99% confident that everyone else here shares that opinion.
You could have fooled me. It certainly didn't look like there was any sort of consensus on anything, what with everyone arguing radically different definitions of the very basic framework of what you were talking about to everyone else before I even came back into the thread...
Anyway, I have to wonder what, exactly, your metrics for high and low fantasy are that a world where it rains the blood of sentient species that causes plagues of undead to roam the land while creatures made literally out of shadows phase in and out of reality and entities of elemental magical forces made literally out of fire and shaped like three-eyed mastadons that fly and spit webs aren't quite divorced enough from reality to quite earn the designation "high fantasy" from you, but, well, I guess that just gets to my next point:
Putting aside that chemosynthetic organisms will, definitionally, not be "plants", I'm not sure why you would come to that conclusion, exactly. You seem to be arguing that "fantasy levels" mean it goes "no fictional creatures" -> "fictional plants" -> "fictional plants and animals" -> "some presumably even more wacky stuff not yet in DF".
I'm just not seeing how you expect these "fantasy sliders" to play out. From a purely rational ecological standpoint, if you accept a break from reality to allow for a magical ecosystem, then it is actually even less believable to make only half the ecosystem appear, but function as though the whole ecosystem was in existence. (That is, presuming you aren't talking about a whole new set of "cavern flora" that is created solely for one gradient of the fantasy sliders, which I doubt Toady would be willing to spend the effort building.) If there's a magic mushroom world, then having animals, even if they're generally inexplicable animals like an elk bird, makes perfect sense when you already accept the break with reality of mushrooms just plain being able to rapidly grow huge in caverns with no access to surface energy.
And once again, the chemosynthetic magma "plants" idea is actually more fantastic and belief-straining than magic is, because it is impossible in ways that break far more physical laws than merely saying there's some magic energy field they can synthesize energy from.
From an internally-rational worldbuilding standpoint, a "fantasy slider" makes more sense being a set of assumptions you can turn on in the world before it is made, like "are there patches of good and evil land where there might be clouds of zombifying dust?" than "select the number of trophic levels in the cavern layer".
From a programming standpoint, I'm expecting Toady is going to be far more direct about what each "slider" will do. Rather than a single "how fantastic" slider, I'm expecting it's actually going to just be more of the same worldgen options we've been getting that sets whether the game makes up procedural creatures like FBs, Night Creatures, Secrets, procedural metals, etc. The new fantasy setting is likely to mostly just be individual settings for how the myth selection works, and if you want to have realistic underground, you already have those settings right now: Just turn off all the caverns, the magma sea, HFS, FBs, various megabeasts, semimegabeasts, night creatures, secrets, region magical alignment, and comment out dwarves, goblins, kobolds, and elves.
The game doesn't have a capacity for recognizing the physically impossible, which leads to unintentional breaks with reality like the mechanisms Expwnent mentioned. Hence, even a "no fantasy setting" is going to still rely upon raws where you can make a civilization of telekinetic horseshoe crabs made of spongecake if you want to in your "no fantasy" world, just because the game doesn't know how to stop you.
A fantasy land where there are just specific types of "plants" based upon chemosynthesis is just the fantasy land variety you're specifically hunting for, which would, honestly, be the place for modding, since I honestly don't think most people want what you are arguing for, nor is Toady likely to supply it.