First thing you should do is forget about going to full frame for the time being. If and when that day comes, you can simply sell off any lenses that are "crop only" and upgrade as needed.
You don't appear to have much need for a wider lens, if you are considering replacing an 18-55 with a 16-35 and state that the 18-55 is one of your lesser used lenses. If you want to learn to shoot wide, that would change things. But if it's just not something you do a lot, back off on what you'll spend for the wider end of things.
Because you have a fast prime, you can probably get by fine with some f4 or slower lenses.
So I'd suggest:
Canon EF-S 15-85mm. This is a fine "walk around lens". The widest one available. You might never need wider, unless you change your shooting styles.
Canon EF 70-200/4 IS to replace the longer tele zoom. If 200mm isn't enough reach, get a 1.4X teleconverter to use with it.
It would be a fairly versatile and simple kit:
15-85mm (USM focus and stabilized)
70-200/4 IS (USM focus and stabilized)
and your current 50/1.8 (tho you might want to upgrade that to the 50/1.4 USM eventually)
You can get both the above for less than the cost of the 70-200/2.8 IS Mark II alone.
Alternatively, if you wanted to start shooting wide more often, I'd recommend the Canon EF-S 10-22mm. Those have been discounted pretty heavily lately and become fairly affordable.
If you get a wider lens like the 10-22mm, you might consider a different mid-range zoom to better complement it. For example, so many have been sold as "kit" lenses (which it really isn't), here in the U.S. the EF 28-135 IS can be easily found on the used market for bargain prices. A premium alternative (better build, but not particularly better performance) is the 24-105L.
If you shoot a lot with the 300mm end of your tele zoom, you might enjoy more reach. The most affordable longer lens is the Sigma 120-400 OS HSM. It's currently on sale, too (I suspect Sigma will be introducing a new one soon).
So you might end up with a kit that looks like:
10-22mm (yes, it's crop only)
28-135 or 24-105 (both with USM and stabilized)
120-400 (HSM, which is similar to Canon USM, and stabilized)
and your current 50/1.8
You mention using your current tele zoom a lot now for macro work. You could continue to do that with the 70-200... perhaps adding a set of Kenko macro extenson tubes to your kit so you can get a bit higher magnification. Or, you might consider a true macro lens if it's something you shoot a lot. The Canon EF-S 60mm is compact and pretty affordable. An alternative is the Tamron 60/2.0... a full stop faster makes it good for portraits (maybe replacing your 50/1.8), but it's not fast focusing so wouldn't be very useful for any sort of sports/action shooting (use the 70-200 for that).
Eventually you may want to consider a shorter focal length fast prime, too, such as a 28/1.8 or Sigma 30/1.4.
Don't bankrupt yourself by going out and getting everything at once or buy buying for full frame usage when you may not ever need that. With good lenses, today's crop sensor cameras are very capable.