The "mega driver" concept basically comes down to bundling multiple drivers/components into a single shared library. By doing so, all of the Mesa symbols don't need to be made public (as is the case now) and it offers the potential for (small) performance wins by being able to apply Link-Time Optimizations (LTO) across the entire stack rather than just on the existing per-driver files.
Emil Velikov posted on Tuesday his "Mega drivers galore" patch-set of 23 patches that refactor the Gallium3D DRI targets into a single library/provider. All of the Gallium3D DRI targets from the three Radeon drivers to the experimental Intel i915 and ILO Gallium3D drivers to Freedreno and VMware's VMWgfx drivers are covered by this work.
Velikov wrote on the Mesa mailing list, "Things works like a charm for nouveau and swrast, and testing on other platforms is greatly appreciated...I would like to get this reviewed/pushed over the next month, although
that depends on the number of bugs that come up with the previous batch."