By Maximum Reich, I mean one where Germany annexes Belgium, the Netherlands, Luxembourg, large parts of northern and eastern France, most of Switzerland, Denmark, Norway, Sweden, Poland, bits of northern Yugoslavia, and the USSR out to the Urals. Probably ASB, but let's allow it for the basis of discussion. Eventually, the Nazi Party (probably for economic reasons, because good luck on that front with your ideology of choice) loses control and is replaced. What happens now? It's 2020 and the non-German 'Germanic' areas have undergone several generations of brutal assimilation. Slavs have been assimilated, murdered or expelled, but aside from western and central Poland and maybe the Baltic areas, the Greater East is fairly thinly populated by 'Germanic' settlers concentrated in cities and towns instead of a flood of yeoman farmers tilling the land.
How much toothpaste can be put back in the tube? Can the rump-USSR/Russia reestablish control over its pre-1941 territories? What about the smaller countries with their tiny, unknown governments in exile?
If this Greater German Reich will have nuclear weapons--and especially lots of them--then I really don't see this rump-USSR/Russia actually being able to do anything about this. After all, it's certainly not going to be willing to risk a nuclear war and thus MAD in order to try reacquiring its old homeland (however attached and sentimental to it the Russians might still be). Even the Palestinians would probably be unwilling to risk this in real life if they actually had nuclear weapons. So, ultimately, the status quo in the East is unfortunately going to be likely to remain for a very long time--possibly even permanently. Obviously if the rump-USSR/Russia won't be able to restore its old borders, then neither will countries such as Poland (due to them having much less people than the rump-USSR/Russia has). What I
could see would be a movement among extreme German left-wingers to allow for a mass Slavic "right of return" to Eastern Europe, but just like advocacy for a mass Palestinian "right of return" to Israel proper is considered a fringe movement in Israel in real life (at least among Israeli Jews), such a mass Slavic "right of return" movement to Eastern Europe would likewise probably be considered a fringe movement in Greater Germany in this scenario--at least among Greater Germany's German population, who are likely to be the ones who are going to make the decisions in regards to this.
The one thing that I could see is a post-Nazi German regime slowing down the rate of Slavic expulsions due to the fact that it might very well become aware that it might need to keep
some Slavic manpower in the farms of Eastern Europe in order to prevent those farms from completely collapsing. As you said, some Germans might move to the cities and suburbs of Eastern Europe, but probably not to the farms of Eastern Europe unless forced to do so at gunpoint--or unless they will move there in order to be farm managers as opposed to farm workers (since being farm workers would probably be perceived as being a job for "those lowly Slavs"--not for "Aryan Ubermensch" like the Germans).
Another thing that I want to point out here is that while the farms and rural areas of Eastern Europe are likely to remain relatively empty (with the exception of the bare minimum of Slavic manpower that's actually necessary to sustain things such as farms, foresting industries, et cetera), over the centuries, Germany's birth rate should gradually increase due to "breeders" becoming a larger and larger percentage of Germany's total population. (You can actually see this pattern occurring among Israeli Jews even right now as their high-fertility groups appear to become a larger and larger percentage of their total population with each and every generation.) Unlike Israel (with its extremely small amount of
Lebensraum), Greater Germany will certainly be (like both Canada and the United States of America) in an especially favorable situation in regards to this due to all of the vast
Lebensraum that it will have at its disposal in this scenario. So, over the centuries, you might very well see tens of millions or even hundreds of millions of Germans live in Eastern Europe--but again, primarily in the cities and suburbs rather than in rural areas. This certainly won't happen by the year 2020 (since there will be nowhere near enough time for this to occur yet)--but maybe it will happen by the year 2500 or the year 3000. Seriously.