I thought it could be interesting to centralize the questions I’m most often asked, and of course try to answer them. Here we go!
Will Sigma make a full-frame 24-70mm f/2 lens?
A Sigma 24-70mm f/2 DG lens has been rumored since the introduction of a certain fixed-aperture APS-C zoom. All wishful thinking aside, can we realistically expect it?
Let’s start with a quote from Sigma’s CEO Kazuto Yamaki regarding the strategy against Canon and Nikon:
The basic idea is better optics, and with an affordable price — that’s the basic idea. It depends on the product, of course, but we are trying our best to achieve that goal. 
Now apply that goal to a “mere” 24-70mm f/2.8 lens, and Sigma has already a big challenge to face. In particular, the Canon 24-70mm f/2.8L II USM is very hard to beat. Reaching the same optical quality but selling the lens at a lower price would be a great achievement.
Going from f/2.8 to f/2 is not just twice as hard as the single stop difference suggests, especially when you have to deal with a zoom that goes from very wide angle to short tele. Some aberrations and other optical nasties increase tremendously.
In the end, it would be impossible to come up with something that is not compromised in some way, or obnoxiously large and expensive. What about a shorter zoom range? Well, it would have to be significantly shorter to help, to a point that the lens would lose most of its raison d’etre.
Speaking of which, Kazuto Yamaki was once asked about a possible full-frame 24-50mm f/1.8 lens. See what he replied:
We might do, but not with the quality that we would like or at least not with good performance at maximum aperture. 
And the interviewer to add:
None of the current 24-70mm f/2.8 zooms are excellent to the level he aspires to achieve. Only the latest Canon version, he says bluntly when talking about competition, is really good. 
Overall, I think it says it all about the odds of ever seeing a Sigma 24-70mm f/2 DG lens. If f/2.8 is challenging, f/2 is simply a no-go.
Any news for a Sigma 135mm f/2 (or f/1.8) Art?
Not really news, but it should indeed be in development… and maybe even at a late stage. Kazuto Yamaki stated something interesting about 135mm prime lenses.
There have been a lot of requests concerning that type of lens. It used to be a very popular device; now it is not so popular anymore. Still there seem to be an increase of interest in it and, as I’ve already said, we want to have a full line-up of lenses so definitely I would like to launch it as soon as possible and include in our offer. 
Encouraging, isn’t it? Well, this was said two years ago, so either Sigma is taking the time to make a fantastic lens, or they changed their plans since then.
That would be unfortunate. In the current lens line-up, only zooms cover this focal length, among which a non-Global Vision 70-200mm f/2.8 and a Global Vision 120-300mm f/2.8 monster that is unlikely to attract many portrait shooters… but none of them is fast enough anyway. The 85mm focal length is served with a good and recent EX lens. So, all things considered, releasing a 135mm f/2 before a new 85mm f/1.4 makes a lot of sense.
Will Sigma make full-frame E-mount lenses, and/or offer their DSLR lenses in E mount?
While this is not a definitive answer to the first part of the question, the below quote from Kazuto Yamaki (September 2014) makes something clear:
For now, we have no development plan for this series. 
Committing to the A7 series means R&D for a brand new, dedicated lens line. Probably not the kind of investment Sigma is willing to make, at least not for now, and the recent interviews with Mr Yamaki don’t show any evidence of Sigma changing their plans. In any case, we’re not close to seeing full-frame E-mount Sigma lenses.
Offering the existing DSLR lenses in E mount could appear as a solution, but Sigma is not particularly keen to do so.
Will Sigma make a Sony A-mount version of the 150-600mm f/5-6.3 Sports?
It’s not planned, so… don’t count on it.
Sigma makes efforts to make most of the Global Vision glass for Sony DSLRs. However, it seems that producing A-mount versions of the 150-600mm S (and 120-300mm S) would be a burden, requiring too many mechanical changes to make it worth it, financially speaking.
Additionally, Sony didn’t show great commitment to their Alpha DSLRs lately, which is not encouraging for third-party manufacturers.
Will Sigma keep using the 1:1:1 Merrill-type Foveon sensor?
I don’t think they will. When trying to think from Sigma’s standpoint, this definitely sounds like a step backwards. Sigma and their Foveon division have been working on the “more advanced” Quattro sensor for years, so they most probably stopped improving what is now the older Foveon.
 Interview with Imaging Resource — April 2014
 Interview with Quesabesde (in Spanish) — October 2014
 Interview with LensTip — November 2012
 Interview with Focus Numerique (in French) — September 2014