...No it isn't. You don't seem to know what makes a Sequel. If they wanted to they could make a Sequel of any Pokemon Generation that they wanted to. There are no special rules to making a sequel.
There was no prior knowledge of a region outside of Kanto when Johto and gen 2 was made. There was no prior knowledge that B2W2 would come out. There was nothing connecting them from then until the sequels came out.
All a sequel is, is a continuation of the story. ALL the Pokemon games CLOSE the story at the end of their respective titles, so everyone is wrong in relation to the story for ANY of the generations.
Kanto's story was done. They closed it with Giovanni's leaving after the Gym Battle.
Johto's reinclusion of Team Rocket was pretty random at the time, but an amazing addition to the story set. Again, the story itself closed after you beat Team Rocket and beat Red.
Hoenn's story closed after you defeated the legend. Both "Evil" (I have in quotes cause Magma and Aqua were really just extreme PETA/Naturalists. They were hardly evil unlike the other three teams.) teams recognized the errors of their ways and simply disbanded.
Sinnoh's story ends after you beat the Box Legend at the top of Spear Pillar cause that's when you beat sense into Cyrus and he disappears. Platinum only extends the story slightly to the Battle Island place where the Three commanders simply get bored of what they were doing and just leave.
Unova Pre-B2W2's story ENDED when N recognized the Player's Truth/Ideal and discovers Ghetsis's betrayal. The story for Unova was over as N flew after apologizing and saying he was going look for another method.
Unova B2W2 A spin reintroduction to Plasma who had effectively disbanded and like with all Main bosses, the leader disappeared, only Ghetsis returned. (If you did Looker's side quest at the end of B/W all of the Sages minus Ghetsis were arrested so there was full closure for the story. They revived the story and added an annex to it with Kyurem who wasn't part of the story from the original and not related to the two box legends in the original set of games.
They revived two teams already who's stories were closed at the end of their respective games. They can EASILY BS a reason to revive Magma/Aqua with different goal because unlike B/W you were not Forced to CATCH the Mascot meaning that they could have left after being defeated, which they do if you choose not to catch them. They could even bring in another team for the sequels that are after Rayquaza and attempt to awaken it by awakening Groudon/Kyogre and repeat Aqua/Magma's steps to awakening the Contenental and Deep Sea Pokemon.
There are a multitude of possibilities to continue Kyogre and Groudon's battle. Heck if they wanted two it can just be two warring factions who want to see which is better, land or sea. A set of teams that fight for the love of fighting and don't care who gets hurt along the way.
Sequels are easily doable just as much as remakes are. And no they don't have to come right after their original games. Many companies can make games far apart and on different systems if they wanted. Look at FF12. It had a sequel for the Nintendo DS. Legend of Zelda's time line is sporadic and doesn't follow game release. The newest game is the Prelude to all current Zelda games so far and the one before that was the last installment to one branch created by Ocarina of Time.
Now do I believe we'd get a sequel instead of a remake? No
Would I like a sequel instead of a remake? No not really
Do I want a Remake at all? Eventually yes, but long after the DS series is dead. Cause then I can get a RF/LG rehash as well.
Do I think there is a Remake on its way? Yes. Not this year, but Next year if a new Generation doesn't rear its ugly head.
Things I'd like to see in the Remake:
Secret Bases with better Decorating ability. R/S/E's Secret Bases were awesome and easily the thing I liked the most next to Pokemon Contests. I was disappointed in the limitation for the Secret bases though and hope they approve upon it. Though seeing as Decorating was removed from G/S's remake (which greatly disappointed me) I have high doubts my favorite system will return.
Contests. IMPROVED Contests. Like more artistic Flare in it if at all possible. Loved the contests. Was so sad how it devolved in Emerald to just Lilycove, and Sinnoh's Contests only being in Hearthome was disheartening as well. Full removal of the Contest stats in B/W and B2W2 leaves me to believe that like with Secret Bases, Contests may not make a reappearence and would be replaced by something else.
What I would Not like to reappear:
Battle Frontier Tents. Useless. That's all they were.
Kinda pretty much it.