I have the 18-55 and 55-200 kit lens. I'm also looking at getting the 18-140 or 18-200
Do you find it challenging or problematic to change between the two lenses you have? The two lenses I use most myself are basically in the 16-67mm and 100-400mm ranges as it applies to your camera, and I rarely, if ever, miss a shot because of having the wrong lens on the camera. I plan ahead, think about what kind of photo I want to get and what opportunities may present themselves, so that the step of changing lenses doesn't occur while the action is unfolding.
If you do find it problematic, and would rather have one lens to cover both ranges, then replacing them with the 18-140mm can be a good option for you. Otherwise, stick with the two-zoom-lens solution; it's versatile and lightweight, and pretty darn good.
I'm thinking about getting the 35mm (52mm equivalent) for my trip. I think it owuld be great on the streets but i also want to be able to shoot landscapes and small architecture.
You have this focal length covered in your 18-55mm lens. You can try setting your lens to 35mm or thereabouts, maybe putting a piece of tape to prevent the zoom ring from turning, and then go out and use it on the street for a day's shooting. Even if it's a boring street in your hometown, just to get a feel of the focal length and the use of a prime lens. I'm sure it can't be a nice feeling, to buy a prime lens only to find out you don't like shooting with that particular focal length.
What the prime will give you over the 18-55mm zoom lens at that focal length is the maximum aperture. Forget about other arguments you hear online, about sharpness and optical quality, or that somehow it magically turns you into a better photographer. The maximum aperture is the first and biggest difference to note. It lets you use faster shutter speeds and/or lower ISO, and it gives a shallower depth of field. If those are things you need in your photography, then by all means get the lens (if you like its focal length). If you don't need those, it won't really be useful to you, so you'll probably end up using the 18-55mm zoom lens a whole lot more.
My absolute favorite DX lens is the Nikon 18-70; this can be had for a song on the used market and has excellent IQ. Add to this a 70-300 for reach, and perhaps a 35mm f1.8 for low light and you'd have pretty much all bases covered.
Well, the OP already has the 18-55mm and 55-200mm lenses. I don't think replacing them with the 18-70mm and a 70-300mm will be very beneficial.