Oh, the "all regions in one game" thing again. Time to drag out my bullet list...
*100 levels divided by 5 regions means that each region can only cover an average of 20 levels. Even Johto, which had the smallest level range, still covered up to 50. To fit within the 100 level cap, the regions will have to be as dumbed down as Kanto in G/S/C
*Or the game could force you to completely abandon your team of six Pokemon which you invested hours into raising and planning out members, movesets, etc. and make you start over. In which case, what's the point of having all the regions in one game if you can't carry over your Pokemon? That essentially makes it five separate games on one cartridge/disc/etc. That doesn't make sense either.
*What if you don't particularly care for one or more regions? For me, that's Hoenn. For some of my friends, it's Unova. Why buy a game if part of it includes a place you don't like?
*How would all five regions be connected in a coherent storyline? Or, would it be, as I mentioned above, like five completely different games on one cart? Who would be the overall champ of all five regions (please for the love of God don't say Red...)? What about the bad guys? What group could be large enough to operate in all five locales?
*Would there be two versions of this so-called "ultimate" game? If so, how could they be "ultimate" if you can't complete the Pokedex in either? If not, it's breaking a tried and true Pokemon tradition.
There, I think that covers all of why such a game is highly unlikely. And why if it somehow does see the light of day, I won't be buying it.