For VC4, my big push recently has been to support some sort of panel. Panels are really big with Raspberry Pi users, and it's the primary complaint I hear about the open driver. The official 7" DSI touchscreen seems like the most promising device to support, since it doesn't hog all your GPIOs (letting me use my serial console) and it's quite popular.
Unfortunately, DSI isn't going well. The DSI0 peripheral is responding to me, but while I can read from DSI1 it won't respond to any writes. DSI1 is, unfortunately, the one that the RPi exposes on its DSI connector. (Note: this debugging is with the panel up and running from the firmware's boot configuration). Debug code is at drm-vc4-dsi-boot
So, since DSI1's not cooperating, I switched tasks. I had also picked up a DPI panel using the Adafruit Kippah, and a little SPI-driven panel. I hadn't started with DPI because hogging all the GPIOs makes kernel debugging a mostly black box experience. The upside is that DPI is crazy simple -- set the GPIOs muxes to output from DPI, set one register in DPI, and use the same pixelvalve setup from before. I was surprised when 2 days in I got display output. Here it is, running HDMI and DPI at the same time:
Expect patches soon on a mailing list near you. Until then, it's at drm-vc4-dpi-boot