You know, thinking about it from a story perspective, I'm not so sure Sombra precisely NEEDED more screentime. He was a different kind of villain than any we've gotten in the past. He was more a threatening presence, a danger that had to be stopped before it started because, if it did start, it would already be too late. Like Sauron, from The Lord of the Rings.
Overall, Sombra's my new favorite MLP villain. Even without the staggering power of Nightmare Moon or Discord, or an army supporting him like Queen Chrysalis, he came closer to victory than any of them through intelligence and planning. Literally the only reason he lost was because Spike insisted on going with Twilight to find the Crystal Heart.
But anyway, Sombra's awesome... and as for this story, well....