For south potential VB route:
1. Bangalore( yesvantpur) to Madgaon- around 639km can be clocked in 8 hours again by VB after doubling and electrification is completed till hubbali or Goa with stops at arsikere, hubbali.
2. Chennai to kacheguda/secunderabad - around 715 km can be again be done in 8 hours via vijaywada with stops at gudur, vijaywada and maybe at warangal.
3....
more... Chennai to Ernakulam- around 700km can be done in 8 hours, with stops at Salem, Erode, Coimbatore.
4. Bangalore to hyderabad as mentioned above- around 668km in 8 hours with stops at dharmavaram, guntakal, kurnool, mahbubnagar.
Changes with stops can be considered but these are the potential long distance routes in South India. The 8 hour or lesser time will only be possible after doubling and electrification is some streches and most importantly, VB should get heavy priority even more than raajdhani if the need arises in these routes to achieve 8 hour mark.