Not a bad idea at all of a merged into one counterpoint team for the X-Men, especially if they are going for one title Uncanny X-Men for awhile. The Horseman sort of work as a sort term opponents of the X-Men, but their goals make Thanos seem reasonable so they never work well in the longer run as good antagonists.
Once the line branches out into multiple books again it might make sense to split them up into two separate teams say Brotherhood and Acolytes or Hellfire could be a replacement for one of them with Magneto in more of a godfather then direct leadership role over them. I made my list a few years ago of a rebooted Acolytes, but in retrospect it has too much a 1990s-2000s flavor.
I would say mix and match members of what one might see as as traditional Brotherhood, Acolytes, and Hellfire team the way you have into one team and with the line being reduced to one book for likely some time keep the antagonist team to one for awhile, before making it more broad in time with say mutants wanting the domination of mankind having a schism with those who want a more mutant separatist/nationalist venture.