If I were you, I wouldn't get any L lens. There are a lot of very good, non-L lenses, too. Don't get hung up on the stupid red stripe... You can take great photos with many lenses that don't have it.
Looking at what you've got, I'd say shoot with that until you can identify a specific need, then look for the lens to fill that need. In your kit, I like the 28-135 (it's able to make images that are impossible to distinguish from 24-70L or 24-105L). Get the hood for it if you don't have it. In fact, get the hood for all your lenses, if you don't have them. And get in the habit of using them.
I'd trade the 50/1.8 for a 50/1.4. The f1.4 is simply a better lens, without being huge, heavy or terribly expensive. It's also reasonably fast focusing.
And I'd want a wider lens.... I use a Tokina 12-24 and like it. Again, not expensive.... And it's built like an L, in case you are wondering. Very similar shape, weight and feel to a 17-35/2.8L I used some years ago, in fact.
You have made no mention of what you shoot and what you feel is lacking. So beyond that, it's hard to recommend much. Personally I need longer teles a lot for the type of shooting I do, so I use 70-200/2.8 IS, 300/4 IS and 300/2.8 IS quite a bit. But that doesn't mean I should recommend any of them to you.
The 135/2L was a "must have" lens for me as soon as I added a full frame camera to my kit. But when I shot crop sensor cameras I didn't need it and was happy with 85/1.8.
I do shoot macros and other close up work, so have four lenses for that purpose. But I'd only recommend the 60/2.8 EF-S or 100/2.8 non-IS as relatively versatile lenses, that can double nicely for other purposes. Both are not L's.
