Beyond Mesa 7.4 we have learned some details as to what's next: merging Gallium3D to Mesa's master branch. Gallium3D, the new graphics architecture developed by Tungsten Graphics, has been in development for quite a while but is nearing a point of stabilization. If all goes according to plan, Gallium3D will see the light of day in Mesa 7.5. Brian Paul announced on the Mesa3D development mailing list that the gallium-0.2 branch will be merged to master following the Mesa 7.4 branching.
In somewhat related news, talks have revived about replacing portions of C code in Mesa with C++. That discussion can be followed in this thread.