I'm a bit late to this one, but my thoughts on this is kind of the same as they've been on both Fantasy and 40k: there is no lack of stories to tell from these universes, but it's difficult to make these stories feel really impactful without warping the universes themselves a lot.
Star Wars, Star Trek, Lord of the Rings, etc. These were all universes that were built from a central narrative with key characters and outward, and as such they had the opportunity to completely change the universes they were in a profound and meaningful way. In the case of 40k and AoS you get the opposite, and the established setting can quickly become a bit of a straightjacket for any new ideas. Modern Star Wars spin-off stories suffer from this too.
For GW's properties, Age of Sigmar suffer most from this, perhaps, since its story is still so much in flux, that you haven't really got any stand-out figures to put into a show and root for. Original creations for the show are fine, of course, but it's likely you'll end up with a band of nobodies doing stuff that in the grand scheme of things just feels kinda pointless.
An example on how to utterly drop the ball on this is the Ultramarines movie of course, a movie about a bunch of nobodies no one cares about, on a mission no one cares about that displays absolutely nothing of the 40k universe's interesting features, with an outcome that had exactly zero impact on anything. That's fine for a throwaway licensed novel, but not for something as costly as a space opera movie.
On the other hand, GW have shown willingness to mess with the status quo lately, so perhaps Age of Sigmar is exactly the setting to get a tie-in TV show or movie, which could actually impact the fluff being written to some degree. That could be neat.
I'm not so sure about putting Space Marines and Stormcast in the main roles though. They are... not extremely relatable or interesting people. Perhaps one or two Stormcast characters, but not an entire squad of them all being serious and dutiful and dour and frankly kinda boring. Put some of them together with a rag-tag band of misfits under whatever is the AoS equivalent of an Inquisitor or Rogue Trader, and you've got yourself some interesting dynamics and story-seeds. Then trickle out just how incredibly badass and messed up the Stormcast are over some episodes, or later in the movie, when there's been some character development.