I concur with the 90. Besides fall off, keep in mind that wide angle lenses get harder to see on the ground glass, especially in the corners. Not impossible though, and with a good darkcloth when shooting outside during daylight hours, it usually isn't a problem. With a 90mm lens, the falloff won't be enough to mess with a center filter. Below that, and you start to take your chances.
I went with a Fujinon 90/5.6. You might consider a 90/8, as they're smaller, usually cheaper, and you rarely need the extra speed for landscapes, other than focusing. Plus, the filters will be cheaper. I also have a Schneider SA 65/8, but don't use it very often. I doesn't have any room for movements, and requires a center filter, but the lens itself isn't too expensive, which helps with the price of a center filter, if you can find one. More often than not, it's too wide. But there are times when only it will do, so it's worth having. But I wouldn't get one until after you've bought a 90mm, as that is far more useful.
I'd also recommend a 150/5.6, as they're cheap and very versatile. It's probably my most used lens. I have a few 210's, but rarely use them. Many of my 210's are over 100 years old, so I use them more for the effects they provide than the focal length. It's just a weird focal length for me that's usually acceptable, but rarely ever my best option. My third most used lens for landscapes is a 360mm Tele-Xenon by Schneider. The telephoto design helps with my field and press cameras, and it's a fairly decent lens. Although, the lens is pretty bit, and it does require a stable tripod and large filters. Still, it's a good value for a longer lens. There are also some 400mm+ telephotos, and some have interchangeable lenses to get you multiple lengths. Though those tend to be pretty pricey. But if you got the dough, that's the way I'd go.
So I'd start off with a 90, 150, and 360 to begin with. Since already have a 210, I might go for the 90 and 360 first, as the 150mm won't give you much difference from the 210mm. Also, a 135mm might make more sense for you than the 150mm with a 210mm already in your bag. I have a 135mm, and don't use it that often. It's kind of like the 210mm to me, in that it's an in-between length. But if I didn't have a 150mm, then I'd probably reach for the 210mm and 135mm fairly often.