“Reassembling the Russian Empire” today would mean subjugating a lot of sovereign nations, including, for example, Poland, the Balt republics and Finland. What was possible, and even desirable in some cases for the Tsar and the healthy Russian nation of before 1917 would not be possible for a KGB apparatchik leading, with mixed success, a weak and diminishing remnant of the Russian people. You are describing the absolute nightmare scenario.
Would he stop at Crimea? It primarily depends on the deterrence that NATO would be willing to provide. At this point he is poised to invade the eastern parts of Ukraine. Hopefully, that would go badly enough for him to lose appetite for more.
Now, do I believe that Putin seeks to physically reestablish the Russian Empire as it was in 1914? No, I believe the man is far too intelligent to harbor such a fantasy.
What I do think he wants to establish some sort of “protectorate” over Belarus, Ukraine, the Caucasian, and the Baltic states. As to Russia's relationship to the Central Asian states, I think a “protectorate” is less probable.
What this “protectorate” would entail, I don't know; and I think its form would vary from country to country.
I believe he will let Poland and the other east European states go their own way, as attempting to control them would only arouse intense antagonism in the West.
I believe Putin wants to establish some sort of trading alliance/bloc with China. Russian energy in exchange for Chinese money would go a long way to modernizing the Russian economy. I think Putin sees this sort of bloc as a Eurasian counterweight to the maritime EU/NATO alliance.
Will this happen? I don't know, but none of it is outside the realm of possibility. If attempted, its success would depend on Putin recognition of limits; that is, his acceptance that none of it could be established except over a long period of time. All he can do is to take the first small bites - such as Crimea - and leave the rest of the meal to his successors.
Never forget that old truism, “Russia is never as strong as it looks or a weak as it looks.”