Germany and Japan pay the 2nd and 3rd highest share of the UN budget, and were only excluded from having a permanent seat because the UN was founded in 1945.
Granted, it would make more sense to have a single EU seat, or to abolish veto power altogether, but working within the system, I'd give permanent seats to India, Japan, and Germany. Brazil, Indonesia, Pakistan, and Nigeria would be next on the docket.
Nigeria is far too weak both economically and as a state. Giving a seat to the African Union would make more sense.