I'd like to point out a few things:
Re: Lunarian + water-themed plot theory, Konohana-Sakuyahime is a water deity, according to Keine. If we consider the mallet, (sentient) rice cake, moon rabbit enemy, and connection to the moon, it's possible---as previous posters mentioned---that Chang'e is involved or relevant. If she is, then we would have a connection with water and (her presumable) immortality, i.e., the sisters Sakuyahime and Iwanagahime, goddess of immortality. Interestingly, Yatsugatake, Gensokyo's highest point and where the latter resides, would be somewhat unaffected if the land were to be flooded.
Re: Theme of time, especially the past, and Lunatic Kingdom; I'll speculate that the Lunatic Kingdom was the Lunar Capital. Eirin said in the first chapter of CiLR that "[...] after Princess Kaguya's great crime and subsequent exile to Earth, the moon changed enormously," which, to me, hints at great political upheavals. Also, the major events that happened during this era, e.g., Yukari's invasion and Kaguya's exile, became catalysts for subsequent ones, i.e., Eirin's betrayal, Mokou's immortality, IN, SSiB (+ relevant parts of Bougetsushou), and possibly one more (LoLK?). From a certain perspective, one could think of these as the Old Kingdom's "legacy."
A minor thought: if time manipulation is somehow involved, and the moon's power is to bring back the past, then Eirin's counter of enabling one to experience the future makes sense (to me, at least *sweatdrops*).
Another minor (cracky) thought: one of LoLK's spoiler for ULiL, following the moon + past + water route, is that Iwakasa's men died either by spontaneous human combustion, or by a fiery being from which Mokou got her power.... On further thought, it could have been both... hmm...
Before, my intuition kept on telling me that ZUN would soon return to his moon saga, but I always thought that it was just my wishful thinking. It appears that I need to give my guts (heh) more credit than I thought. :V I also find it fascinating that, prior to this, there have been significant moon-related occurrences, e.g., TARC Miscellaneous Questions thread's moon discussions and Hifuu Club's episode. In conclusion, I'm very happy with this announcement. (More brain food for me!)
(I wanted to post this days ago, but stuff happened. Oh well, better late than never.)