The plan was to stall (and survive) Sauron's attacks.
That was basically what everyone did in the Third Age: Holding off Sauron's attacks. Never once, since the Last Alliance, was Mordor attacked directly. (Until the end of the Third Age obviously). It would have been too foolhardy to do so.
Before Mordor was re-established, Sauron's stronghold was Dol Guldur. The plan was to defeat that stronghold. (Refer below)
First off, let me clarify your point on the White Council. Their purpose of being formed was mainly to defeat the power in Dol Guldur, not Mordor in general.
2460 The Watchful Peace ends. Sauron returns with increased strength to Dol Guldur.
2463 The White Council is formed. About this time Deagol ...
Lord of the Rings: The Return of the King - Appendix B
As seen from the quote, the White Council was formed due to the return of Sauron to Dol Guldur. Its main purpose was to destroy the forces of Sauron in Dol Guldur.
Back to the plan; Sauron was really powerful by the end of the Third Age, having bred countless orcs and recruiting lots of Men. The plan, if the Ring was not found, would have been to stall as long as possible.
There were a few main (powerful) kingdoms which could stall Sauron:
- Gondor (Minas Tirith+Dol Amroth+Lossarnach+...)
- Thranduil's realm in Mirkwood
- Grey Havens
These kingdoms would have stalled Sauron as long as possible, especially Gondor. This is suggested during the Council of Elrond;
At once Boromir stood up, tall and proud, before them. 'Give me leave, Master Elrond,' said he, 'first to say more of Gondor; for verily from the land of Gondor I am come. And it would be well for all to know what passes there. For few, I deem, know of our deeds, and therefore guess little of their peril, if we should fail at last.
'Believe not that in the land of Gondor the blood of Númenor is spent, nor all its pride and dignity forgotten. By our valour the wild folk of the East are still restrained, and the terror of Morgul kept at bay; and thus alone are peace and freedom maintained in the lands behind us, bulwark of the West. But if the passages of the River should be won, what then?
Therefore, should the Ring have not been found, the main plan for the Peoples of Middle-Earth would've been to stall Sauron, even if they could not.
'I know little of Iarwain save the name,' said Galdor; 'but Glorfindel, I think, is right. Power to defy our Enemy is not in him, unless such power is in the earth itself. And yet we see that Sauron can torture and destroy the very hills. What power still remains lies with us, here in Imladris, or with Círdan at the Havens, or in Lórien. But have they the strength, have we here the strength to withstand the Enemy, the coming of Sauron at the last, when all else is overthrown?'
'I have not the strength,' said Elrond; 'neither have they.'
Fortunately, the One Ring was found, so the new plan was: to send the Ring into Mount Doom whilst the rest of the kingdoms tried to stall.