With all the new alt-art we're getting I've found myself trying to fill in trades with fancy art of cards I already have.
I believe a "is alternate art" filter would be very usable, anyone else?

Yes. I would love this feature. I often want to search my basic lands to see which full-arts I have.

While the utility of the idea is sound, in implementation it couldn't be as simple as "is alternate art". Technically, four artworks of a basic Plains in a set are all alternate arts, but that's obviously not what you want. A flag like "is fancy" is closer to what you're after, but the problem with that is it's too vague, not easily definable. What's considered "fancy"?

The filter would more likely need to be one of those "is any of" / "is none of" filters, with a list of possible treatments. Then you could have things like "showcase", "extended", "borderless", "full art", "retro frame", etc. all of which have relatively clear definitions. This then allows each user to subjectively choose what sort of "fancy" they're after, and it also makes the filter more extensible (i.e. future-proof).