Poststadion already has BAK, Lichtenberg and a few others playing there so it's not an option either.
Viktoria recently bought some land in Mariendorf but that's mainly for the clubhouse and some training pitches. There's a lot of criticism that they never tackled the stadium issue even though the rules were pretty clear.
They could also potentially build a temporary stadium...it's been done before (Düsseldorf, Wiesbaden). The main problem will always remain the lack of space in this city. Hertha did a lot of research (albeit for a larger stadium) and they didn't find many suitable spots. Their ideal solution was building a new stadium right next to the Olympiastadion and the next best choice was already in Ludwigsfelde which is outside of Berlin and was therefore rejected by the fans. Someone in the senate did bring up a spot in Tegel but if I recall correctly others rightfully pointed out that it already had been promised to someone else.
While you're surely right, it does sound ironic talking about Berlin, when no other big city is (or used to be) so littered with vast, or even abandoned, spaces.
And there's two defunct airport areas in the city now (touching Tempelhof is a no-go, of course, and thankfully so).
I don't know who it is that Tegel was promised to, but I remember seeing a billboard erected there advertising this kind of Brave New World-ish campus of, I don't know, co-working office spaces, startup park, pre-furnished city apartments and recreational area to be developed. That was way years ago though, before the anewed delays in Schönefeld and the prolongation of Tegel's activity. No idea whether the project still lives.
It's just hard to believe that, if anything, it couldn't be there, or in the vicinity, that a space could be found. There's this whole area, between Rehberge and towards the airport, where there's the Zentraler Festplatz, then there's some giant ADAC trainig space, or parking lot?, vast areas of empty asphalt. Next to the A111 autobahn, too.
The Olympic stadium, I assume the whole Olympic area would be under monument protection, no? Could be difficult to just build a stadium right next to it, and then behind it is this woody hill isn't there?
But I'm not a city planner. And there really is a lot of space in Brandenburg. Teltow, Grossbeeren, Ludwigsfelde.. sounds alright for a Hertha-Viktoria-Stadion to me.
Are Viktoria planning to move the club out of Lichterfelde then, to Mariendorf?