A ZAG influencer shared info about the upcoming Tokyo and Rio specials:
x.com
They're expected to release next year, between Seasons 6 and 7.
However, they'll be set chronologically earlier in the series (maybe around Season 5) so they will use the old character designs (with the new animation, of course).
Gabriel might make an appearance in them.
For 'Tokyo', the scenes in the city will be in 2D, with the rest in 3D.
Aside from Stellar Force, Kagami/Ryuko will make an appearance in 'Tokyo'.
Luka might appear in 'Rio'.