How do you improve on the 135L?
I guess you do something like this :
The Zeiss from all accounts is an amazing lens, what we have come to expect from Zeiss, they make few poor Lenses, but.
For Canon & Nikon, this particular 135f/2, though sublime, lacks auto focus, so it's all manual, which is Ok as well, but the Canon 135L is a pretty hard act to follow considering it's maybe 98% as good as the Zeiss 135f/2, and has auto focus, on a Canon body.
I haven't actually used the Zeiss 135f/2, but I do own the Canon 135f/2, and it's pretty good.
I own the Sigma Art 35f/1.4 as well as the Canon 35f/1.4 L, and both are excellent Lenses, the Sigma may have the sharper image, but the Canon has the better Bokah, it's all subjective to the individuals taste & preference, and wallet.
I think also "distant stars" original "How do you improve on the 135L" didn't need to inspire the "Canon Fan Boy" tirade that followed, this is an open forum, people are allowed to post their views, and within normal reasonable bounds, shouldn't expect to be hammered for them.