The lenses that you decide to use are ultimately a very personal decision. Some people like primes, others zooms, others all-in-one-zooms. Pending your shooting style, whether or not you get the shot that you want will be determined by the focal length that you have available along with the aperture you have available on that particular lens.
Some people here would prioritize never changing the lens while traveling - thus using a super zoom. The trade off of this decision being decreased image quality and a need to push the ISO due to super-z's usually being in the f/5.6 range or worse.
A more professional kit would be something like a 16-35 f/2.8, 24-70 f/2.8, and 70-200 f/2.8. 3 lenses with a massive range all available at f/2.8. Obviously, a kit like this is expensive and heavy.
If one wanted to pair this down a bit, you could do something like a 16-35 f/4, 24-105 f/4, and 70-200 f/4. Same range, sacrificing the aperture for a lighter weight kit.
And of course, there are many more variations of this using other zooms with more closed apertures or adding primes in to fill gaps or add low light flexibility. Ultimately, the choice is yours.
However, for landscapes, I would personally sacrifice carrying everything I own to simply have my 16-35 f/4 and 100mm x 100mm filter kit. Why? Because the wide angle suits my style and, for me, the importance of a polarizer and ND's and GND's for landscape work cannot be overstated.
Within your existing kit, you obviously have a gap to fill at less than 55mm. How you fill that is completely up to you and your budget (pro zoom, consumer zoom, prime, etc). I would encourage you to budget in a good polarizer, though, at minimum.