[Beleg]

Sorry, I don't have the original version of the Silmirillion next to me. I'll try a quick explaination of the extract (end of the Quenta Silmarillion)

"Quant à Morgoth, les Valar le jetèrent au-delà des Remparts du Monde par la Porte de la Nuit, dans le vide Eternel, et ils font à jamais garder ces remparts [...] Morgoth Bauglir [...] avait semés dans le coeur des Elfes et des Humains étaient une semence qui ne pouvait ni mourir ni être détruite. Elle renaît et bourgeonne de temps à autre pour donner des fruits noirs, et cela jusqu'aux derniers jours".
You'll be able to find easily this part before Akalabeth.

It is about:

1- The end of Morgoth, who's thrown out of the World by the Valar, into the Void.

2- The mischief he spread and left in this world. It is a malice that never dies and comes back up sometimes, until the end.

I think Sauron cannot come back. There is no possibility.
see all the topics above ==> When the One Ring is destroyed, it happens to him almost the same thing than to Morgoth. As Sauron put a great part of his powers into the ring, once it's destroyed, his body is cast off from Arda through the Gates of the Night (or annihilated, I don't know). His remaining power doesn't allow him to reshape, as it is not whole. For me, Sauron is not living in this world anymore.

If Sauron doesn't come back, it doesn't avoid other evil to take shape, as it is said at the end of the Quenta Silmarillion. Maybe the mischief passed into the Humans (it already happens, the Haradrims are an example). There are corrupted things left into the World when the last Elves came back to Valinor at the end of the 3rd Age. When Morgoth was banned, there were servants of him hiding in the World.
Sauron succeeded to Morgoth, another will succeed to Sauron.

I think the end of the 3rd Age means that it's the end of Sauron. (it's a parallel) With the new age, another ennemy will arise. This one is just the germ of what the two previous Dark Lords sowed.
There is no need to have Sauron coming back all the time. Things have to change. The World belongs to the Men, so let it be so with the evil, the darkness belongs to another Sauron's or Morgoth servant.

It was a pain to explain that in English.