1) Despite what he's saying, he will have the constitution changed if he's asked enough by the masses (I think this can be finally ruled out now that he's announced his support for Medvedev becoming his successor)
2) He'll become Prime Minister and either have the constitution changed to move power from the office of president to that of prime minister or have his successor step down in favour of himself after a fig leaf period, technically avoiding a third consecutive term.
3) He'll rule from behind the scenes, maybe taking over the management of an influential company like Gazprom.
The snag of both 2) and 3) is that he has to rely on his sucessor to adhere to whatever agreement was struck in advance and not to stab Putin in the back as soon as he passes on the keys of the Kremlin.
So, today I read something that made me think that Putin may consider a different option:
BELARUS DENIES REPORTS THAT UNION STATE CONSTITUTION WILL BE SIGNED SOON.
Pavel Lyohki, a spokesman for President Lukashenka, on December 7 denied media reports that Lukashenka and Russian President Vladimir Putin will sign the Constitutional Act of the Belarusian-Russian Union State during their mid-December meeting in Minsk, Belapan reported. Lyohki's statement contradicts reports aired by Russia's Ekho Moskvy radio that the presidents will soon sign the Constitutional Act. "This obscure information came as a surprise to Belarus," Lyohki said. "Since it appeared in the Russian media, I advise you to approach the Russian side for comment," he added. Pavel Borodin, Russia's state secretary for the Belarusian-Russian Union State, earlier said that the meeting between Lukashenka and Putin will focus on a draft of a Constitutional Act for the proposed union state.
In the twilight years of the Yeltsin administration, Belarussian president Lukashenko promoted the Belorussian-Russian Union; not only to save his economically declining country from bankruptcy, but also with the ambition to become Russian president by stealth. Putin never was a fan of this Union, although the Kremlin kept paying lip service to the Union as a final goal, and some Union institutions exist, largely unnoticed by the world.
So any movement here could be an indicator that Putin took a leaf out of Lukashenko's script - becoming Russian president again through the backdoor of the Union presidency.
After this idea came to me, I googled a bit and found a denial distributed by RIA - so if there's smoke, there may as well be a fire?
Вкратце (предполагаю, что вы все знаете версии насчёт того, как В. В. сможет оставаться у власти, "от и до"), я прочитал вышеуказанную заметку и думал - может быть, он пойдёт другим путём? У Александра Григорьевича с этим вариантом в своё время не получилось, но Путин, конечно, это - другое дело.
РИА уже отвергает этот вариант - значит, не один я набрел на эту мысль...