What about continuing I-97 south and having it join I-95 near Richmond? It isn't like we can't use the capacity.
The problem when attempting to discuss an 'outer crossing' for the Potomac is that MD has always favored an eastern bypass and Virginia has always wanted a western bypass
Even so, there are plenty of people along the 301 corridor who are opposed to improvements to that corridor as it would induce even more growth than the already-insane development along the corridor that's already happened, especially between Waldorf and La Plata.
Realistically, I think the best one can expect in the next 20 years is a replacement of the Nice Bridge (eliminating that bottleneck) and completing the freeway conversion of MD 5 between the 301 split and the Beltway. Aside from the Nice Bridge, I highly doubt you'll see any improvements to 301 proper.
Which is both a shame and no big deal. It's a shame in that it would give long-distance travelers a better alternative to 95. It's "no big deal" because long-distance travelers are really not a large chunk of I-95 DC-area traffic...you might see a little improvement through NoVA on the weekends, but effectively zero improvement to the weekday slogs through Woodbridge/Dale City, the Wilson Bridge, and Greenbelt.