It is very unlikely for a Crystal remake, as said many times before. For one, if there might have been one, then at least a quarter of all the new features added now might not have been implemented into HeartGold and SoulSilver, but rather left for the Crystal remake. Not to mention, new hidden areas, like the possible Battle Frontier or Tower could have waited for the Crystal remake as well. Seeing as Yellow never had a remade version for the GameBoy Advance is what makes this Crystal remake idea unlikely.
However it is never impossible, and you never know that maybe in the next ten years, rather than remaking FireRed and LeafGreen for the latest system at that time, they could remake Yellow instead. The same could happen for Crystal as well, but for now HeartGold and SoulSilver seems to have it all. Not to forget, having a Crystal version isn't something needed, seeing as what HeartGold and SoulSilver is bringing in seems very satisfying. Plus, a remake of Crystal would only delay the waited fifth generation to come in like a year or two at some point.