Can there be "one day becoming a fully-awakened Buddha" after Parinibbana? The Buddha declare to venerable Sâriputra in the Lotus Sutra that he will become a Buddha one day as it read below, but Sariputta has achieved Parinibbana. Can there be becoming a fully-awakened Buddha one day after Parinibbana? 


  ...Again, Sâriputra, at a future period, after innumerable,
  inconceivable, immeasurable Æons, when thou shalt have learnt the true
  law of hundred thousand myriads of kotis of Tathâgatas, showed
  devotion in various ways, and achieved the present Bodhisattva-course,
  thou shalt become in the world a Tathâgata, &amp;c., named Padmaprabha,
  endowed with science and conduct, a Sugata, a knower of the world, an
  unsurpassed tamer of men, a master of gods and men, a Lord Buddha.
I have no idea why this question get so horribly down voted. I suspect it's due to angry Theravadins who see this as being against Theravada orthodoxy of final permanent Nibanna.

The fact is the Lotus Sutra aims at deliberately over turning such conception about Arahatship,  Nirvana and Buddhahood. Mahayana Buddhism relies on Prajnaparamita sutras as to establish that there is no final permanent nirvana nor the fact that Arhats are merely to remain as Arhats, but should aspire to Buddhahood with the full capacity of a Buddha. The core doctrine used to explain this view is through the understanding of dependent generation, which consequentially result in the impermanence and hence emptiness and non dual nature of any phenomena.

In the Lotus Sutra, it is therefore posited that Sariputra (Sariputta) shall one day attain full and complete unsurpassed awakening as a completely realized Buddha. The sutra further establish that only with such view, would they be True Arhats.

The sutra further explained that it is through skillful means that the Buddha teach sentient beings to give up their desires and attain Nirvana. Because most beings want to end their suffering but are afraid of taking on the challenge to become a Buddha. This is compared to the story of a rich man whose son have ran away from home, and now becomes a poor labourer. Now the poor man eventually reunite but no longer recognize his own father and is afraid of him thinking he's unworthy of being equal to the rich man. The rich man assigned the poor son various tasks until he gained his confidence and can truly inherit his father's legacy. The rich man is the Buddha who assigned the poor man that is sentient beings work, I.e. the way to end suffering Nibanna. But they should not be content with merely Nirvana but should aspire to be truly equal with the rich man e.g. the Buddha.