Imagine your typical pokemon game for Nintendo DS. Now imagine these improvements:
-Every area from every game is accessible, much in the same way that Kanto is accessible in HG/SS.
-One could start in any hometown (ie. Pallet, Newbark, Littleroot, etc.) and would get starters corresponding to that version.
-All pokemon would be obtainable. And I mean all of them, not some stupid event schedule thing that us Canadians wouldn't even have a glimmer of hope at attending. Events would be able to take place, but they would be for trainers to get together and battle or provide a surplus of something, maybe an extra masterball or uber-poffins or something along those lines.
-The storyline would be somewhat the same as the games combined with a few connections here and there with increasing difficulty based on the levels of the pokemon in the party.
-Battle Frontier as well as many other Tournament-based challenges would be accessible via the different regions.
-Now, you might be asking: How would you travel to and from regions so seamlessly? My answer: The Boat system. You've likely already seen the travel to Sevii Islands in LG/FR or to the Battle Frontier from Snowpoint in D/P/P. Hook all those boat docks and ports and such together and voila, travel system.
-Feel free to share your thoughts on the matter.