Hey, thanks for doing this!
My question is how developed are your ideas of what Spren look like and how they behave? I would love to see some illustrations of them in the future.
There's some early illustrations of them in the original pitch package, where they look very Miyazaki if I say so myself, but my understanding of them has widened considerably in the time since then.
I usually envision (and illustrate) them as ghostly, glowing shapes of infinite variety, a lot like the classical interpretation of a hologram. To the best of my understanding, spren appear to be made of stormlight when they're made apparent at all. Some have more detailed features than others, but I don't think any appear solid unless they actually manifest in the Physical Realm, and the only manifestations we know of are Shardblades (living blades have freedom to reshape though, so who knows what that could mean).
As to their details, if you take the few descriptors that Brandon gives and let your imagination run, you probably can't go wrong... even among the spren, two of the same type might look very different in detail. And an awakened spren has a lot of mutability, Syl is changing shape all the time.
As with all concepts, the text rules. So angerspren is gonna look like blood-red pools boiling up from the ground, and gloryspren will look like tiny translucent globes of golden light. Brandon wrote it, so that's what it is.
But you can translate "blood-red pools [of light] boiling up from the ground" in a few creative ways, and any of them might be equally valid. As the Interlude suggests, the appearance of spren is kinda quantum... fluid until/unless observed. I have a lot of fun seeing how creative I can get with the description while making sure I'm staying true to the text.