Way back when, in the dark days of the last administration, before there was any indication that the US might get semi-serious about high-speed rail, I spent some time making a fantasy TGV network map for France. Why? Because.
The excellent base map I used came from Railways Through Europe. Existing Ligne à Grande Vitesse are solid red. Lines that RFF is considering are dashed red. My proposed lines (though I admit that I stole a number of them from LGV2030) are in dashed cyan. The little bits of dashed pink are lignes classiques that I think need to be electrified and/or double-tracked to support trains coming off the LGVs.
The main goal of my plan was to provide fast, direct inter-regional services. So far, the TGV network has been centered on Paris. This makes sense, of course, as the Paris aire urbaine contains about 19% of the country's population. But I think economic development away from the capital would be more vigorous if the TGV network was more of a mesh than a spoke-and-wheel arrangement.
On the Paris area map I added one new color: yellow for a cross-city tunnel linking the Gare Montparnasse with Gare du Nord and Gare de L'Est. As of now, there's no good way to run trains through or around Paris from the southwest to the north and east. Travelers going through Paris in those directions have to cross the city on the subway or otherwise to continue on their journey. Travelers going through Boston face similar difficulties. A tunnel would solve the problem in both cities, though at great expense.