I think it would be neat if there was only one terrain. Now hear my out on this first as I know on the surface it sounds rather like a bad idea.
I don't even know how I got this idea, I guess I'm just rather fond of having one huge landscape rather than small, segregated biomes (I guess you could call them that). The world just seems rather small when you have multiple landscapes as each province never really truly captures the feeling of a certain terrain. I'll give two example of some of the larger, more expansive provinces - Snowpeak and the Gerudo Desert (both from Twilight Princess). In terms of this series, these terrains were rather large but somehow still failed to capture what a desert of isolated snowy peak should feel like. There is a reason for this and I guess it's because Zelda games don't really go for mass scale. So by having 6/7/8 different terrains your essentially making each terrain smaller as you can only have a limited land mass.
My way of countering what I said above is to have one dominant terrain. I'll go with the two examples I used above, being a desert and a snowy terrain. Imagine Hyrule, or maybe an entirely new land, encompassed in snow or set within an expansive desert. This way you can capture the atmosphere that said terrains should have and truly showcase the full extent of what an overworld should feel like. I tend to feel slightly claustrophobic in Zelda games as you feel somewhat trapped within a miniature world that contains smaller lands. There is usually no separation or realistic transition between one terrain to the next; you can go from a volcanic terrain to a forest, a giant lake to a desert, a snowy mountain to a Kingdom and so on.
Of course you could still add some diversity within this set landscape as if you look at any large mass of land (on our planet) you will notice that it's not consistent throughout and there is actually a lot of diversity. If you can create that within a game - having one terrain and still keep that sense of fresh discovery - then I think this idea could be pretty darn awesome.