It's important to remember that Rockslide didn't simply comeback as an alternate version of him. He was suffering from psychosis due to having multiple versions of himself in his head. So it's not likely to be a proccess that they'll repeat willingly
Not really. Jordan White said in a podcast that the FCBD comic had to be made very early on because of printing stuff and that they put characters there that probably would be in a mission like that, but they didn't know yet. By the time they started writing the crossover, they decided it was Rockslide, Glob was never supposed to go really. And it makes sense, since Rockslide is in the previous issues of HiX-men talking to Summoner about his weakness and Glob would make no sense in the crossover.
Jordan even said that Tini wrote the cards texts for the FCBD issue and she said "I'm writing this, but it is probably going to change, right?". The story was only in first stages.