Countdown to Destruction is a two parter and it only brought two actresses back who had been on the show very recently (the rest were rubber suits and voice actors), the biggest part of the season was still a Megaranger adaptation. This Hexagon idea would require tons of original footage, it's completely at odds with the Hurricanger footage, the storyline as presented requires specific rangers to appear when not a single actor had been approached about his availability etc.
Look at Power Rangers in Space, it featured the search for Zordon, it brought back some older characters but those were small parts, most of the time the show revolved around the new villains, the Andros/Astronema story and the other current rangers. Hexagon sounds like Countdown to Destruction stretched over an entire season, there's so much wrapping up and fanservice going on that the new rangers seem like an afterthought (did you notice that they are barely mentioned?). Power Rangers should always be about the current team, you can throw in references to the past, you can bring back old characters but only to seve the current characters stories.
That's why Gokaiger worked, they brought back countless characters but at its core the series was about the Gokaiger themselves, it was their story, they learned and grew, the old characters just helped them out along the way. Hexagon is too in love with the franchise's past and puts too much focus on it, that what makes it sound like dumb fanfic to me, it's a fanboys wet dream but not a realistic approach to an adaptation of Hurricanger.