I am not one to say what it should be, really; no matter what I would think of what they do next, it is better that MLP not fall into a rigid formula that suffocates any new or unique ideas. I would like them to reintroduce some more classic characters however; especially Grogar, because we never truly got to see him after G1.
That said, I do have a sneaking suspicion that it will be a reboot of Generation 4. Incredible as it may seem to us dinosaurs, Friendship is Magic is well over a decade old now, and you can already see little flickers of nostalgia for it here or there among the young adults who watched it as children. Furthermore, G5 has carried over several major elements from G4 as well, and is quite literally a direct sequel to it. The baseline of G4 is something they seem very hesitant to let go of so far, and I doubt this will change. Finally, the iconography of G4 in general still defines this franchise; G5 made the right move by introducing new characters I believe, but I don't think Izzy or Sunny will ever be on the level of Pinkie or Rainbow Dash, for better or worse.
The iconic nature of G4, and its target demographic coming of age as potential new fans and customers, especially if they have children of their own, makes me think they are going to lean hard into G4 nostalgia once G5 is over. The corporate aversion to risk, the ripe timing, and our quite nostalgia-heavy pop culture (or whatever remains of it) paints a picture of a G4 reboot to me. Not to say I am FOR it necessarily; I could honestly say the opposite. Nonetheless, I am quite sure there will be an appetite for more of what G4 offered one day on the horizon, and Hasbro will be keen to pounce on the opportunity.