On top of that, look at the Pokemon anime. A common question concerning it was what the heck is up with this huge gap between Snowpoint and Sunnyshore (and as a result it not looking like Sinnoh will be over until anytime before summer at least). The anime getting word of the 5th gen coming in around the fall of this year when they were producing those intermediate episodes would explain that. Hearing things would be coming up that quick, they debated between whether it would be a good idea to blaze through the rest of Sinnoh and do a quickie Battle Frontier arc or just hold out Sinnoh until the 5th gen comes. Feeling that although lagging through Sinnoh wouldn't be the best possible thing that could happen, it definitely beats rushed development for the rest of Sinnoh and a short frontier arc (if it was short, they wouldn't have time to properly market the saga, and thus it wouldn't be worth it), and thus they went for the one long arc over two short ones, in order to fill up that gap of time.
Beyond that, what are the other possibilities? Crystal remake? Why then would they have bothered putting Eusine in HG/SS at all if that were the case? He wasn't in the original G/S, and they didn't need him. Doing so would only make them needlessly think of something new to make Crystal stand out again, which wouldn't be needed if they just stuck with Eusine. The only way his presence in HG/SS makes sense is if Game Freak had absolutely no plans whatsoever to actually remake Crystal itself, and thus stuck him and some other features from Crystal in there to make up for the fact.
There's also the fact of the upcoming Pokemon Movie: Phantom Champion Z. The interesting thing about this movie is of course it's title, which some fans have interpreted as possibly referring to a new Pokemon, especially since that unannounced merchandise showed up. However, the title is interesting in more ways than that. It bears a striking resemblance to the original title of a certain other movie: movie 2. Before Lugia was revealed, it was also called something akin to "Phantom Champion" in the title. If this isn't just a coincidence, and it is indeed an advertisement for a legendary from Gen 5 as it seems to be, then Gen 5 will be getting plenty of hype from that movie alone (and since CoroCoro is going to be revealing what Phantom Champion Z is next month, if it is a Gen 5 Pokemon, that would add on several more months of hype for the games, which is plenty enough, especially with Nintendo's more recent business model of trying to cut down on the promotion a bit and only announcing games closer to their release time).
I suppose one could try saying that it's a replacement for Crystal, or rather a new addition to the GS saga instead. But again, they already had Crystal to work with, and a Crystal remake would have been a surefire seller. If they were truly interested in going that route, they would have just remade Crystal. But as said above, if they were going to do that, they wouldn't have placed the defining differences of Crystal into HG/SS.
Other than that, since this is Game Freak, there's really only the possibility of Gen 3 remakes left. This is an actual realistic possibility. However, that would put Game Freak into a bit of a crunch in Gen 5, as that would force them to remake DP then, which would be way too soon for that. Having Gen 3 remakes in Gen 5 would keep the pacing nice, with an appropriately aged generation getting remade in the successive generations, and not leave them with something as new as DP getting remade. Plus, there have been numerous events for the majority of the RS legends in DP already, such as the Regi trio, Jirachi, and even Deoxys. The Titan trios themselves were made catchable in HG/SS, along with the Lati twins. All things considered, it's just too soon for RS remakes right now.
Putting all of these pieces together, the 5th gen is looking like the most likely outcome to me right now (although, I must say, even I didn't expect it this soon: I expected it next fall at the earliest. Still, I can't make else out of this, so that's what I going with for now).
