Thing is, both Minato and Kushina were coming thanks to the different triggers. 8-Tailed Kurama and Naruto taking Kurama's power. I seriously think now, what could be the trigger for Hagoromo to appear? A new Juubi Jinchuuriki in this world wasn't, the God Tree alive again wasn't, revival of the Juubi wasn't, but it may be of Juubi's complete revival. Yeah, that might be. Come to think of it, he said that the full revival of the Juubi will signal the end of the world. I think that Madara won't be able to restrain Juubi and it'll go on a rampage. Now imagine a rampaging Juubi, splitting moutains, swallowing oceans, etc. End of the world indeed.
That's the moment I see of Hagoromo to return. Other than this, I can see his body sealed somewhere holy. Like the moon. Knowing he's dying, he may have got to the moon he created and seal his body there. A powerful body like his would have been stolen if it were buried on the planet. I can see his body being summoned on the Earth and revived the moment the moon is shown the Tsukuyomi.
