I think the P5, will forever be the P5 until the dissolution of UN.
China will never allow Japan (through veto or other means) to be part of the P5, even though Japan is historically the second largest financial contributor to UN budget. Now because of the increasing closeness of Japan and India and India with US (Japan and India both vouch for each other for the UNSC seat back a few years ago), which threatens China's strategic outlook, China will forever blocking both's entry into UNSC.
At least Japan has a good reason being it is the second largest contribution UN, India on the other hand is not even in top 17, its contribution to UN is so insignificant even a bankrupt Spain, a small third world economy like Mexico contributed more than India. Even Brazil another BRICS nation has contributed more.
To be honest, I think the whole UNSC permanent member status is just silly in a democratic institution and the fundamental failure of the architecture of UN. If India, Japan or Germany is allow into P5, ALL other countries will want to be in for the special privileges. Why give special decision-reversing-game-changing rights to only very selected countries? If UN really wants to live up to its function, it needs to do away with the "Permanent member in UNSC" and the associated special rights afforded these nations. Just revoke P5 status or even better the whole UNSC and have all security issues talk in General Assembly only.