Hi and welcome to POTN,
There are 18-200 and 18-270 lenses available, that might be convenient for travel... But often this type of "do everything" lens doesn't do any of it particularly well. You'll have to decide for yourself, if their image quality is up to your expectations and if any other compromises they make are acceptible.
A more reasonable range is the Canon 18-135mm that's often offered now as a kit lens. Nice range for a walkaround lens and mid-priced, but 135 often isn't enough reach for wildlife.
The problem is your favorite subjects. You really probably need two lenses minimum.... a wider one for landscape/architecture... a telephoto for wildlife. "Nature" can be many things from scenic vistas to high magnification macros.
Cheapest solution, continue using the 55-250 (which is quite compact considering) replace the 18-55 (if it's the older non-IS version, the newer IS version is better optically anyway). But you are still "stuck" with two lenses (though, after all, that's the whole point of an interchangeable lens SLR camera... the ability to change lenses to best meet different situations).
The premium and pricier replacement would be the 17-55/2.8 IS.
If you found yourself coming up short on width with the 18mm, you might want wider and the 17-55 is only slightly. Canon 15-85 is a high quality, general purpose, reasonably compact but not cheap alternative that gives a bit wider angle of view.