One thing's certain: the story won't revolve around any of the original cast. We won't see Barney, Robin, Marshall, Lily, Ted, OR the mother.
Don't plan on any crossovers either.
According to The Wrap,
"If this new show goes next year, it will be a brand new show. It will be a brand new story," Bays said. "We kind of didn't want it to have any feeling of trying to resuscitate something that has already ended."
Thomas said they wished they could use Cristin Miloti who plays the (so far) nameless mother this season, but that they want this show to be its own thing without any ties to the original (well, other than the title).
"In the fall, if we succeed with the spinoff, we will have to make our case to the audience in a whole new way, with a whole new world, with whole new characters. And that feels right; we want to honor '
If you think you'll just be seeing the same exact story from a woman's point of view, Bays hinted the show will have a twist that makes HIMYF "a totally different show."