Huge post right here ... I compiled everything Alec said about a sequel:
Thank you for all the work, Chibi!
I still think the theme he has for the game he's currently working on is similar to Aquaria. Young female protagonist, formerly manipulated, rebels against her creator, and has body-altering powers.
Will the monkeys be in the sequel? (If you're planning on one, that is)....
You play as a monkey in the sequel. 
I bet he's not kidding either.

Humans are monkeys. Maybe you get to play as Li.
I think there's some confusion here ... lol 
Yeah, if you're talking about a possible sequel, which we're totally not working on right now, but maybe at some point in the future... then yeah, there are some ideas where it would take place. (the secret ending hints at part of it)
Seeing the flying castle, I thought right away about flying forms. That would be fun. Maybe for the 3 games we'll have water, earth, and air. It would be nice if we get a mod-able engine at some point that allows us to make use of all the systems from the previous games. Swimming, flying, and walking, or whatever they end up being.
My 2 cents:
Right now I don't think we could handle working on a sequel, because we really just feel like working on something new.
That said, the sequel would be quite different from the original and involve a number of new ideas and game play concepts, so I don't think it would feel like a rehash either.
I also think it would benefit the sequel if we waited a while and came back to do it at a later point in our lives when we have a different perspective on things. (no idea when that would be) It would be interesting thematically, we could kind of challenge our old ideas.
I'm sad that it will be so long before we get to see more, but I hope Derek intends to return to it as much as Alec does, and I'll be waiting for whenever it happens. I guess it will also give them a chance to improve and learn new things to apply to a new game.
I'm also waiting for the hard-copy game to come out to buy it. I gotta have pretty box art on my shelf. It'll go with my Ryzom and Myst boxes. (and WoW. hahah) Not sure about the soundtrack, but I am interested and do like to support my favorite artists.