Star-X (Super)
Star-X is the Star-Force fusion of Star-A, B, C, and D. Star-X has only been seen twice, both against Seventy-Five. Star-X demonstrated the ability to summon allies to help, though they were all useless against Seventy-Five's Nova form. Star-X gained a super form thanks to Star-C and D's new forms, and it was enough to combat Omega Seventy-Five at 100% (post 80-year revival). 

Further Fusion Speculation

Since Star-K is in Universe 1, it's possible that he could fuse with the other Stars into something else. However, this may not be possible due to the fact that Star-K is an alternate universe version of Star-C. More speculation comes from the fact that Seventy-Five was once "Star-E." It is unknown if he can fuse with the other stars as well, though it's unlikely, due to his very contrasting nature from the other stars.