Sigma spilled out some info in a new interview with Imaging Resource:

APS-C L-mount lenses:
Yamaki said “we will release APS-C sized lenses for L-mount in 2020“. These will be L-mount adapted lenses of their current APS-C lenses for MFT and Sony E-mount

New natively designed lenses for mirrorless:
Yamaki: “brand-new lens designed for the short flange-back system should be available some time this year.”

Sigma’s most selling lenses are…
….”I can’t tell you the exact numbers, but the most popular right now is Canon, and next is Nikon. Sony E-mount is growing”

About the possibility to make Canon RF and Nikon Z lenses:
…”We are still checking their systems, and it’s a bit too early to make comment. But right now, we’re checking the compatibility between our lenses and their mount adapter. We already know that it works almost perfectly, but in some specific settings and [for some] operations, our lens does not work perfectly [with their adapter yet], like continuous burst mode...”

About the Sigma FE lens sales:
…”Surprisingly good. They like the optical performance of those lenses. It’s better than I expected. But there also are some people waiting for the lens design for the short flange-back, so we are also working on it”

Market is shrinking.
Yamaki believes the market will shrink further: “Maybe [down] 20 to 25% from now, that’s my guess.”

DSLR vs Mirrorless:
…”the market is changing from DSLR to mirrorless much quicker than I thought.“…..”Eventually, I think the majority of interchangeable-lens cameras will be mirrorless.“

Dpreview had a chat with Tamron managers and here are some of the info they shared:

Next FE lens:

People really want a telephoto zoom to cover focal lengths longer than 75mm. We’re looking [into] that.

About future Canon RF and Nikon Z lenses:

Yes, we’re also looking at that area. We have to do a lot of research and development into the reverse engineering, because they don’t disclose the details of their systems, so it’s a really hard job for us.

APS-C vs Full Frame:

APS-C is still important to us, but when we think about the [industry], the full-frame market is expanding, so we’re looking at that market first – that’s the first priority. So gradually we’ll create a [full-frame] line and then at another time we can launch more APS-C lenses. The APS-C market is shrinking quite fast.

Dpreview writes:

The APS-C market is shrinking, and Tamron would be irresponsible to pour significant resources into developing lenses for a contracting market. The audience for lenses designed for full-frame mirrorless, on the other hand, is guaranteed to grow over the coming years. With two FE lenses for Sony already on the market, it was reassuring to hear from Tamron’s executives that longer focal lengths are being planned, to round out the lineup.

Next, of course, will be reverse-engineering lenses for the Canon RF and Nikon Z mounts (it seems unlikely that Tamron will bother with L-mount given Sigma’s head start as a member of the alliance). In our conversation, Tamron’s executives reinforced what we’ve been told by other optical engineers (including Sigma’s Kazuto Yamaki) that designing for a longer flange back distance and adapting that design to shorter mounts is the only practical way to create one optical formula for several mounts. As such, it seems possible that before too long, the popular Sony FE-mount 28-75mm F2.8 might form the basis of new standard zooms for Canon and Nikon mirrorless.