Your challenge, should you accept it, is to create a strong German Reich that lasts for 1000 years. After that, you can do what you want with it.
Your only limitations is that this Germany must be a leading nation in European and World affairs, not some tiny state that managed to survive through mere chance of history. Apart from that, be as creative as you like?
What can you come up with?
Rekjavik
No way you can be certain any power will be a major state in a century's time in the industrial age, let alone a millennium. Too many variables even presuming that neither total collapse nor the centre of human activity moving off Earth occurs and I suspect one or the other very likely way before a millennium passes.
However wouldn't be difficult to have a marginally less stupid Hitler/Nazis that perform a bit better in the war against Russia and don't pick a war with the US. The latter may well join the conflict later on but is unlikely to be able to carry through things with the same determination without a German dow following on very quickly from Pearl Harbour. If the Germans win in the east, say by using divide and rule and at least short term acceptance of groups such as the RLA, then a negotiated peace is likely in the 43-44 period.
After this a period of fanaticism leading to the slaughter/expulsion of many of the Slavic population of eastern Europe and European Russia would make room for a large German population to settle the area and give the basis for a long term domination of Europe and western Asian possibly, for the foreseeable future.
After that some relaxation with the collapse of Nazi dogma so that the state isn't crippled by its insanity and you could easily see a very large and powerful state developing that potentially could achieve your target.
There are other, less nasty, ways of achieving your target but this is probably the simplest and most likely to stand a chance of a lasting domination of the sort of state your after.
Steve