Anywhere in the world where I have never been.
Mine is probably a lot more logistically achieveable in practice than a lot of other options here given that it has pretty reasonable infrastructure and is within short budget flight range of me, but I would really like to go to the temperate rainforests of western Scotland and look at the lichens. I am amazed by the richness and the species composition every time I do IDs from there.
For something that must stay in the realm of daydreams but really would be the dream: where I live now but in the Eemian. The straight-tusked elephants would be fun (from a distance), sure, but the real kicker would be the vegetation structure of the landscape. Denmark is so heavily modified by human activity so deeply in time that the “natural” landscape and evolutionary context of our native biota is pretty close to unimaginable.
Gimme Australia or the Amazon. Hands down, amazing places
This is actually totally true. Everywhere IS an iNatting destination.