What do u think will happen once/if the O5 return to the past???

Maybe they go back & because jean shows Xavier what happens in the future he doesn't mindwipe them or maybe he still does. What if with that info Xavier decides to right his wrongs. Showing magento the future causing him to abandon his ways earlier & join the x-men bringing in wanda & peitro. Xavier gets to Emma first or actually convinces her. He would go after wolverine earlier & could probably save Raman & x-23. Legion would have been a student before his powers set in same as proteus. They would ve more prepared for the phoenix force, for sinister, & apocalypse & koarka. Vulcan & his team would have lived. Not saying they would win every battle & be undefeated but they would be a better off team. Xavier would have made better choices in the illuminati because of what he did learn thru jean. Xavier probably would work more closely with the scavengers & SHIELD. magneto would have been given sovereignty over genoesha sooner & under better terms. Maybe wolverine leaves & joins avengers more permanently or even other xmen too. I think the "mutant problem" would have ended sooner before it began making them more mainstream heroes along with the avengers & fantastic four or it might cause the scavengers to never form leaving captain America, ironman hulk & Thor to join SHIELD. Spiderman could join xmen as a mutate. Namor a mutant but focused on his people becoming xaviers ally.... Man I think a lot could come from a revamp if done right.

I've said it before, but I think when the O5 go back we can expect some sort of alternate Universe event for 2-3 months before they return to the normal continuity.

It'll probably be like how Age of Apocalypse showed the importance of Xavier, but instead, like you pointed out, it would show how great the world could be with the O5's knowledge of the future.

Then of course I'm sure they'll find a way to say that this alternate timeline is in someway flawed and they need to fix things again which will result in the return to status quo.

Well, it will totally rule out all the interesting things that have happened to them for the past 5 decades. I imagine it would serve as some sort of reboot.

I honestly wouldn't be surprised at all if after Bendis leaves the book some writer just nullifies the whole story.