auras are a neat idea, but I'm convinced they wouldn't work simply because of the sheer complexity of allomancy and how it's used. To start with, there are 16 allomantically reactive metals. Even cutting it down to just the 11 that are in the first book (thereby painting ourselves into a corner should we want to even consider the second) that's still a lot of unique auras.

Asking the audience to spot the difference at a glance between 11 auras is already a challenge, but now put like 7 of them on a single character burning a bunch of metals at once, while they knife fight in midair surrounded by mist with a second character that is also moving and burning a ton of metals. Not only is it just visual noise, but even if you managed to convey which metals were being burned and the awesome depth of the fights taking place, the glowing blobs of light would just serve to distract from the fact that two people are flying through the mists and knife fighting. With this confetti of colors in motion, would you even notice if one of the characters suddenly lost their "pewter glow"?

Then the other challenge is explaining the glow in the first place. I had a similar thought a while back, and my initial idea was to just explain the glow as how a seeker perceives someone burning metals (which isn't true, but for a film, is a little excusable). But what about fights between mistings? What if there's a smoker present, or the mistborn just isn't burning bronze? Does the audience just not get to know how things are happening, just because there's no seeker? We could just keep the glow on all the time, but then that ruins other things. For example: in vin's battle between cett's hired mistings, the entire tide of the fight goes through a massive upset because vin isn't burning bronze and fails to notice something. If the audience was privy to this information the whole time, nobody would be surprised by this surprise.

But let's say that we do achieve a method of making characters glow with bright colors that manage to communicate the complexity of mistborn fighting. Suddenly someone wants to do a warbreaker movie, where characters actually do radiate light and colors when they have enough breath. Or what about stormlight? Characters in stormlight literally glow, and have to be conscious of how much they glow when being stealthy, but characters in mistborn glow brilliantly but actually they aren't glowing at all, and their glow is actually a totally different glow than biocroma which happens to look exactly the same?

For a movie taking its first step into the cosmere, it seems really irresponsible to just start stepping on the toes of at least two other series.