In general I agree that you should work with what you've got and learn to use it well... find out what you feel is lacking, and then look into adding a bit at a time. Still, it doesn't hurt to think about what you might want, down the road.

The 18-135mm offers a nice range of focal lengths, from moderate wide to moderate telephoto. It's also got Image Stabilization, which will help you get a steady shot with slower shutter speeds that might be necessary in lower light situations, by counteracting slight camera shake. However, IS can't do anything to help with blur due to subject movement at slower shutter speeds. For that, you need to use higher ISOs and faster shutter speeds.

Another factor is focus speed and a lens' tracking ability. The 18-135mm IS uses a micro motor, which isn't particularly fast or good at tracking. You can get the best out of it by using only the center AF point on your camera, which is more sensitive than the other eight AF points. The 18-135mm "STM" has a moderately better AF drive (stepper motor) and any of the Canon USM lenses (or third party equivalents) woudl be be even better yet.

DANCERS = indoor and stage lighting = large aperture lenses and higher ISOs. Learn to use your camera up to ISO 3200 and 6400 (shoot RAW, overexpose slightly, up to +2/3 stop, post process with extra noise reduction applied).

Lenses: EF-S 17-55/2.8, EF 24-70/2.8 II, EF 70-200/2.8 IS II. Fast zooms are bigger, heavier and expensive... and the best any of them can do is f2.8. So you also might want to look at prime lenses such as 35/2 IS, 50/1.4, 85/1.8,135/2. These are one to two stops faster than any zoom, yet more affodable. For dance you will need fast focusing lenses that track well, too... So stick with USM lenses (or third party equivalent such as Sigma HSM or Tamron USD).

WILDLIFE = lots of telephoto reach, though just how much depends upon whether we're photographing elephants or humming birds. The small the critter, likely the longer the focal length that will be needed.

Low light will be less of a concern most of the time, but too slow a lens (not large enough aperture) can at times make for problems, early morning and evening, when wildlife can be more active. It's nice to have on any lens, but I especially encourage image stabilization with longer focal lengths. There are many zooms and primes that you might consider, for example:

Canon 100-400 IS
Sigma 120-400 OS
Sigma 150-500 OS
Tamron 150-600 VC

Canon EF 300/4 IS (works well w/ 1.4X teleconverter too)
Canon EF 400/5.6 (not stabilized)

These are the more affordable long telephotos, but they start at about $900 and go up from there to about $1700.

All the above are USM or third party equivalent, too, which will be useful since wildlife photogrpahy also frequently needs fast focusing and good tracking capabilities.

You get what you pay for (and are willing to carry around)... For example, an arguably more versatile lens than any of the above is the Sigma 120-300/2.8 OS. It's f2.8, so more usable in low light than any of the above. It's also usable with both 1.4X and 2X teleconverters, so can serve as up to an effective 600mm. But it's bigger, heavier and costlier: $3600 for the lens alone. Still, that seems almost cheap in comparison to some of the truly premium lenses such as Canon 500/4 IS Mark II for $10,500 or Canon 200-400/4 IS with built-in 1.4X teleconverter for $11,800.

ARTY ENDEAVORS = ?

There are all sorts of specialized lenses: Very large aperture for shallow depth of field effects. Macro for extreme close-ups. Tilt-Shift for focal plane and perspective control effects. Even soft focus for dreamy portraits and more.

We really need to know more about what you are trying to accomplish, before we can advise.