Restore the former "glory", influence zones and importance of the Soviet Union / now called Russia. Defend his dictatorship against western influence of democracy and self determination of nations. Putin sits on a huge pile of natural resources and he wants to use that as a power instrument. It's the fate of many resource rich countries, that it leads to governments (and helping classes) enriching themselves.
But I don't think Putin is self-destructive / suicidal. There is no hope for controlling a hypothetical conquered European country for any length of time given Russia's present capabilities.
Realistically the only thing Russia can hope for is the preservation of the status quo - keeping Crimea and the land bridge to it.