Trust me, we have thought about them and almost every other skin idea out there. We talk about skin ideas all day every day and we love it! Sharing fan ideas along with our own and having a lot of fun trying to convince others that 'This is the year we finally do flying spaghetti monster Vel'koz'* is an important part of each day, but ultimately there has to be some tactical decision making to ensure that we are doing the best thing for all our players.
We didn't call the Prehistoric skins Jurassic because they aren't museum skeletons. Every time we discuss a skin for a void champion Jurassic comes up and we consider it, and I am pretty sure we'll make Jurassic Kha'zix eventually.
On the subject of Lunar Revel this year, I can't talk about it yet and that is perhaps the source of a lot of frustration (for you and me). A skin like Project: Zed took a really long time to come to fruition, and all the while players on the boards are asking 'why is Zed so unloved?' even though we were working on it. Unfortunately it's the nature of the work, it would be far more disappointing if we talked about it in advance and players got excited and then we had to cancel it for any number of reasons.
On the subject of Nocturne, I love this champ so much! I suggest him in open ideation sessions when appropriate. But if you look at the themes from last year; House of Cards, Dinosaurs, Bildgewater, Academy, RPG, he did't really fit. He could have made an excellent Marauder or Project candidate in my personal opinion, but there were other champions that also fit these themes really well; eg. Warwick hadn't had a skin since 2011 and has never had a legendary, same goes for Master Yi (early 2012).
However there was a huge increase in cadence last year you probably noticed, and only ~20 champions didn't get a skin (which, compared to previous years seems incredible to me) so the chances are, barring current reworks in progress, that those champions will now be a high priority for us.
*Alas, it is not. I'll try again in 2017 :P