We should be able to go to the Sevi Islands, Kanto and Joto. To get to Kanto you need to beat the Sevi Islands and the people there could be trying to start up their own pokemon league and the former elite four member Lorieal (I know i spelled her name wrong) is running it which is why she left the elite four. So to get to kanto you need to defeat the sevi island pokemon league and to get to Joto out just need the 8 kanto badges and then you can go on farther to Mt Silver. After getting all Joto Badges you can fight Gold and Red in a double battle Both Gold and Red will have both 6 pokemon so you would need to take down 12 pokemon from each of them.
I thought this would be a good post game and in both regions it could allow you to obtain more Kanto and Joto pokemon such as syther/ sissor game corner and be able to go to reconizable locations such as Slip.co who lowers security because team rocket just got disbanded and you can explore the business levels and also the Radio tower in Lavender town all floors.
You could catch the 3 legendary birds and Mewtwo and we can use the S.S. Ann to fight trainers like a tornament arena like in the post game from Black and White 2 the Safari zone would be opened up and you can catch a Dragonair and Dratini at this Location and you can go to the newly Built up powrplanet and it would have a basement level where you can catch Zapdose, The Seafome Islands will Have Artacuno and Moltress would be found Inside Mt. Silver. Celibi would be at the shrine and catching Entie Rioku and Suicune (spelling for their names please) would be in Tin Tower and you can fight each one However you have to catch all 3 at once like once you catch one then the next and so one But if you Ko one the other 2 should run off and cant ever be found. If you do get them you can go to Tin tower with the golden Feather to Catch Ho Ho. Also once you get you're Last Joto Badge you can catch Lugia at the Sevi Islands By obtaining the silver wing from professor elm and you can chose one of 3 other starter pokemon from the Kalos region (It could be random) If you lets say picked cyndiqull you can get the Hoen starters if you picked a torchick you get the shino starters if you picked a shino starter you can get one of the unova starters. so this way you can get some of the starers that you didnt pick in reborn added into to you're pokedex.
(I am sorry if some of the names are spelled wrong but i tried to spell them on how they are pronounced in my opinion.)
and yes i put way to much thought into this.