hmmm... you bring up a good point that it doesn't have to be a planet or planetoid. I mean, Asgard in the MCU clearly isn't a traditional "planet," so Muspelheim doesn't have to be either. I think the first image you posted looks pretty awesome.
In the end, I'm all for something that is visually striking and unique. Something that we haven't seen in a film before.
I hope that T:TDW really does give us a glimpse of all the realms, as has been teased. Even if it is just a montage, a cinematic version of this would be awesome: