A meeting is scheduled for widening of a 9.5 mile section to a 4-lane divided facility
http://www.txdot.gov/inside-txdot/get-involved/about/hearings-meetings/dallas/080817.html (http://www.txdot.gov/inside-txdot/get-involved/about/hearings-meetings/dallas/080817.html)
Schematic
http://ftp.dot.state.tx.us/pub/txdot/get-involved/dal/sh121/080817-schematic.pdf (http://ftp.dot.state.tx.us/pub/txdot/get-involved/dal/sh121/080817-schematic.pdf)
Looking at the schematic, sections are built to freeway standards including three grade-separated intersections, and other sections have frontage roads. There are stub-outs where the frontage roads end, suggesting they can or will be extended in the future to upgrade to full freeway standards.
The right-of-way appears to be sufficient along most of the length to accommodate the frontage roads for upgrading to a freeway, but there are sections where the right-of-way is constricted and the frontage roads probably won't fit. So I'm wondering why they don't acquire sufficient right-of-way along the full length for future frontage roads. The Dallas district usually acquires wide rights-of-way, and it's strange that the corridor for this expansion is not consistently at least 300 feet (the usual minimum for a freeway).
Google Earth imagery shows they are also expanding SH 121 between US 75 and the Collin County Outer Loop, but on a narrower right of way, likely an urban arterial instead of a typical rural divided highway. I guess a future freeway would link to US 75 via the Collin County Outer Loop and not directly through Melissa.
Suburban growth appears to be accelerating north of McKinney, but mostly along the US 75 corridor so far. Towns like Melissa, Anna and Van Alstyne have recently built new subdivisions.
They've already begun construction of a grade-separated interchange at SH 160 near the Collin-Fannin county line.