Dear Marco,
Giving advice on choosing a camera is such a boring task! Mostly, it is boring because such is giving advice in general: advice is often given but rarely sought.
For you, however, I shall make an exception!
Now, if I had to start from scratch, I would advise you to consider the following points:
1. in what type of occasions would you take photos (at the beach, at parties, in the streets, during travels, in studio, etc)
2. what would be the end products (photos for Facebook, prints for a gallery, photobooks, commercial catalogues etc)
3. the price you pay
4. the price perceived by others (i.e. the «status symbol»)
However, I am not starting from scratch (in the sense that I already know your photos, and I know you a little!), neither are you (you already own a good camera system). Since you already use a «full frame» DSLR with a decent set of lenses, and you already know you like to take street photos with a zoom, my advice to you will tend to select a camera that, while satisfying your needs, will be a good companion, rather than a replacement, for the system you already own, and complement it.
Your system lacks in one main point (lightness and compactness) and excels in some respects that are not crucial for street photography (low-light scenarios, architecture, product shots in studio); therefore,
I will try and select a system for you that:
a. is suitable for street photography
b. provides an excellent image quality in daylight conditions
c. is compact and light
If you did not already own what you own, and if you were not fond of zoom lenses, I would recommend you what I consider the best digital camera for street photography: a Fujifilm X100T.
However, this is not my advice for you, not only because you seek a zoom, but also because (staying with full frame DSLR) you could get an EOS 6D with its «pancake» lens and obtain a system that, in terms of size and weight, would not be too different.
My advice for you is therefore a digital camera with a small sensor and a long-reach zoom. By choosing a small sensor, you lose in some respects (low light capabilities and shall depth of focus) that in my opinion are rarely crucial for street photography; on the other hand, you gain the capability of using zoom lenses that, in the film era, were unimaginable in this size. So, here is my list for you.
1. Sony RX 100 IV
2. Panasonic LX100
What you might not like at first of these cameras is that they do not have a long telephoto within their zoom lenses; I don’t believe it to be a serious flaw because the image resolution is very high (at low ISO) and you can easily crop.
Now, if you wanted to abandon your EOS system, my advice would be a little different (I would go for a Fujifilm X-system or an Olympus Micro Four Thirds system) but, wanting to keep your 6D and use it for when it really shines, now (November 2015) the two cameras above are my advice for you.
Enjoy it :-)