I'm of the opinion that a new mane cast would be preferable, just so that the new series can be sure it can set itself apart from the previous one. The way they seem to be going about it, it feels more like a bizarro version of G4, which I think will probably turn more people off than a completely new universe would.
Part of my opinion of this comes down to (and I realize I'm going on a teeny bit of a tangent here) how reboots in general have also tended to be not-so-well-received by fans of the original (Powerpuff Girls, TTG, Ben 10, etc). When reboots do work, they tend to be under very special circumstances, like 2017 Ducktales (released a LONG time after the original ended, generally keeping a similar premise to the original and keeping most of the characters roughly the same) and, a recent discovery of mine, the 2019 Carmen Sandiego Netflix reboot (which, despite using the same characters as other installments but slightly redesigned, is based on a franchise that didn't follow a single continued story arc and was already being continually rebooted anyways.) Even then, those series aren't immune to comparisons to the originals. I know MLP is also based on a highly rebootable franchise, but at this point it's so popular and has generated such a deep continuity and dedicated following that rebooting it at this point would be less like rebooting Carmen Sandiego and more like rebooting Gravity Falls (which would no doubt send LOTS of Fallers in a tizzy.)
That said, if they do stick to rebooting the Mane 6, I do think there are things they could do to make it work better- namely, just making the setting and the characters as different as possible from G4. I saw a comment on EqD that had some interesting character ideas, like making Applejack a deputy of a western town, or Twilight a simple book seller. Maybe don't have all 6 be friends from the very start- pair them off into their respective pony races, so you have three different groups of friends to follow (or maybe more). And I'm probably in the minority when I say this, but I'd actually like to see new voice actors for them. Because honestly, my worst case scenario would be if G5 ended up being this "sort of almost G4 but not quite" kind of rehash, basically unable to escape G4's shadow as a result of being too similar. So while my preference would be some sort of continuation of the world of G4, next best thing would just to make it entirely its own thing (heck, you could even imagine it as the same world as G4, just maybe thousands of years into the future, where Equestria has changed).