Here's what happens if DDD isn't included in the next collection.
(Disclaimer: Steve has a PS3 and is new to the Kingdom Hearts series!)
Steve: Well, I've spent hours on 1.5 and 2.5 remixes and covered the plot. Now I'm ready to play Kingdom Hearts 3!
*buys KH3*
Steve: WHY THE HELL CAN'T I UNDERSTAND WHAT'S GOING ON I BOUGHT BOTH STINKING HD COLLECTIONS!
Seriously though, not having DDD in the collection is totally counter-productive. Like sephiroth0812 said, the collections are about
1. Remedy the issue about the story being spread over so many systems.
2. Bring the fanbase up to speed by using a home console to center the series on.
And by the way, the PSP game MGS Peace Walker was released in a PS3 HD collection just 17 months after it's PSP release. DDD has already been out for 8 months. Therefore it wouldn't be unrealistic to expect KH 2.5 HD Remix, including DDD, to be released outside of Japan in time for Christmas this year!
That's very generous, but my point is you can't use the excuse that DDD hasn't been out long enough.