Totally 100% agreed, although it might be a bit difficult to establish another story in the timeline since there is a definite beginning and a definite end. Something between Second Story and TTEOT giving some background on the Aldian/Vendeeni forces would be nice, but I'd be more partial to something between The Last Hope and the original game because I want to see why Welch changed her hair. And the game should be entirely about Welch because she's the single most interesting part of SO lore.
As for other franchises, I'd love to see Valkyrie Profile 3 with Hrist as the main character. She's my absolute favorite character in the series and it's a shame that she'll probably never get her time in the spotlight.