Most of the 700-series was allocated to airliner projects, but not all of it (see Boeing model designations: the 700-series in the "Designation Systems" section of this forum).
The final "7" may have been considered as a lucky streak kind of thing... In the past, the Model 300 had been redesignated as the 307, possibly for the same reason.
At any rate, it became a kind commercial gimmick to memorize the aircraft's name easily, and Boeing even protected all 7*7 designations to prevent any use by other companies.
When the Model 367-80 prototype was considered for production, two variants were planned: the 707 with the "8" shaped fuselage and the 717 with the round fuselage.
The latter was drafted in both airliner and transport version, but in the end, only the transport 717 was built, becoming the C-135 and KC-135 series.
As for future designations beyond "797", I suggest that they will go the Peugeot route: when the 107, 207, 307, 507, 607, 807 and 907 were all allocated (Peugeot's trademark designations are built around a central "0", they created the 1007 (with the double "0") and this became true of several new models (3008, 5008). Probably Boeing will keep the "7" but add two figures in the middle ("7007, 7017, 7027..." or "7007, 7117, 7227...").
