Seems like a lot of the objections against the main candidates (Japan,Germany,India,Brazil) are pretty emotional. E.g. Italy's objection against a german seat is just a defiant reaction to the situation in european politics where France,UK and Germany make the decisions and Italy takes a back seat. (more or less)
Given the facts that new permanent members will be admitted and that Italy will most likely not be one of them, I dont see how the admission of any other candidate instead of Germany would serve Italy better.
The situation with Mexico/Argentina vs. Brazil and S.Korea vs. Japan seems pretty much the same. (Although there's also the issue of the Japan not admitting war crimes in Korea, etc.)
Also, I dont see why one of the new permanent members should necesarily be African. I mean, the main job of the UNSC should be to prevent and solve crisis. So candidates aptitude should be determined by stableness, intl reputation and influence, ability to react to crisis, willingness to use those abilities, etc.
Nigeria for example lacks most of those. Their only plus is their military and their commitment to peacekeeping missions in the region. But internally they have lots of problems, in half of the country sharia courts form the legislature, intl reputation and economic power are mediocre at best. S.Africa seems a little better overall, but they support the Mugabe regime, their military is rather small, GDP/capita ist below 3000$, not much on the pro side.
Judging from these facts, even smaller western countries like Switzerland would be better suited for a permanent seat. (Though Switzerland probably wouldn't accept due to neutrality reasons)