I think its down to what aperture you are chasing and based on I what I have learnt with the kenko 1.4 I would look at avoiding extenders. They aren`t bad especially on bright lenses, 2.8 or better, but generally not worth the effort if fast focusing is a priority.
for aperture------- the 17-55 IS USM 2.8 and 70-200 2.8. But I reckon you will need to add something wider around 10-22 Sigma, Canon Tokina make good ones ( aperture most likely not so important here).
general purpose ------- 15-85 IS USM and 70-300L IS USM ( or 100-400L), should almost get away with the 15-85 and not require a wide angle lens. I would avoid the 70-200L F4 as the 70-300L does it at f5, which is very close to f4 and has more range and better IS. Then use a few primes to make up the stuff in the middle. ie 30 1.4, 50 1.4, 85 1.8, 135 2.0
cheaper general purpose ---------- Tamron 17-50 2.8 and tamron 70-300 SP VC, still leaves wide angle to be considered.
Would like to see the new 100-400L if its like the 70-300L in build quality IS, weather resistance, focus speed and only just a bit longer and heavier without the push pull , it will be on top of my list.
I think for me, on a cropped body the ideal aperture would be f2.8 from around 30 to 130 then f4 through to f5.6 up to 400.
As you already have the 18-135 and the 30 1.4 maybe just add something in the long zoom range for now.
Canon Powershot S95, Canon 6D,7D, Canon 40 2.8 STM, Tamron 24-70 2.8 VC, Canon 17-40 L, Canon 15-85, Canon 85 1.8, Sigma 30 1.4, 50mm 1.8, Canon 100 2.8L Macro, Canon 70-300L +Kenko 1.4 Pro 300DGX, Canon 430EX II and RS 4 Classic