The Germanic symbols, in particular the D rune, are definitely viable alternatives.
Thor's hammer also makes sense here - but it seems to be too concrete, too closely linked to a specific story. I suppose Hitler was deliberately seeking something with more abstract associations (power, vigor).
One symbol not yet mentioned is one the Nazis actually used: the Eagle.
In their own variation of course, but leaned from its association with Germany since the 19th century (because of the glorification of Hohenstauffen).
This again is concrete, but already cooked down sufficiently as a heraldic symbol.
However, it is not really Nazi (or even German) specific.
The Swastika, as the D rune, have the prime advantage that they are completely unknown to the average European of that time, completely abstract, and so can serve as a perfect means of identification for the Nazis, and for association of almost everything you
declare it stands for.
So I think, the Swastika was really a smart choice (as would have been the D rune).
Since Hitler pretty much wanted to remake the German Empire (plus a massive wank), the most likely alternate flag for Nazi Germany would be the flag of the GE, which actually was used from 1933-1935 as Germany's flag until it was banned.
You mean black and white and red?
Think of the 20s: A political party can't just use a former national flag as party symbol.
Moreover, there were enough issues which separated the Nazis from the from the Empire of 1870 - but they decoyed its old supporters by their common hostility to the republic. The Nazis thus attracted particularly military elites and partially businessmen - because they needed them.
But using the same colors is all you can get here. Again, the Nazis went well with something unrepeated, something unmistakable.