A big, big indoor museum/library tileset. It's satisfying smashing up 4000 year old artifacts. Basically make a huge library/museum tileset filled with books, bookshelves, databanks, ladders, stairs, paintings of bald old men and of course a giant book with the title 'JJ2 Source code' that is shut with a giant lock labelled 'Epic Games'. There should be dinosaur skeletons (you should be able to climb onto a dinosaur's, maybe a T-Rex, skeleton's backbone, tail, head, etc), mystical talimans, Egyptian stuff, a JJ1 OEM version inside a vacuum-sealed, laser-guarded and bullet-proof glass box, lots and lots of museum-stuff like big doors, shelves with rocks in it and maybe a chemistry/biology/physics lab filled with experiments like clones, perpetual-motion machines, universal dissolvers, etc. The whole thing should have an old, booky and "educated" atmosphere a la British Museum New York public library and New York Natural History museum. There must be lots of JJ1, 2 and JJA references (I've already mentioned JJ2 source code and JJ1 OEM). Maybe there should be a locked up door that you can't access that says: 'JJ3 - a new exhibition that is coming soon' or just 'JJ3 - P R I V A T E - Authorized staff only!'. There should, of course, be rabbit, turtle, lizard skeletons and skeletons of all of the other creatures/bosses in JJ2, JJ1 and JJA (ah having a huge cameleon skeleton from JJA would be great!). There should also be suckers and floating suckers inside sealed glass containers full of that yellow, preservative liquid, shrunken rabbit heads (that'll be cool!

) and lots and lots of different platforms, statues, marble/metal/stone floors (look at the Haunted tileset and its different floors, stairs and platforms) and crystal chandeliers to climb and walk on. Maybe you could trigger a 100-year machine gun? Riding on a giant magical flying book? Wandering through dark, lonely libray aisles only to found that you're not alone (!)?. Looking up and see giant glass roofs or 19th architecture (I especially like having it in 19th century architecture because it feels comfy

)? Dealing with noxious fumes, live animals/labrats and escaping from a flood of sulphuric acid that was accidentally spilled all over the floor in the lab? Despite popular belief, museums and libraries are FUN places.
I think that this tileset is so amazingly bad because:
1. It's so much more original than most other tilesets. Spaceships? Done that. Ships? Done that. Industrial areas? 7th Lava Fall, Cyberspaz, done that. Giant rabbit? Done that (Inside out, by Fawriel!

). Old and ancient themes? Done that. (Egypt tilesets by Agama and Kejero and Medieval Japanese themes by Splash). No theme at all? Wisety has done most of the tilesets with no themes, or an outrageously crazy one (remember the Fishy tileset?). Snow tileset? Admittedly HH98, Holidaius, Inferno day and night and all the other snow/winter tilesets have their limits, but there are already so many of them. Beach tileset? Same as with the winter tilesets. A World War II tileset? Done that (Wartorn, by Toxic Bunny). There also doesn't seem much point in doing a WWII battlefield tileset since fighting in a WWII urban area (hence my suggestion of Wartorn) is more interesting. But yes, a battlefield tileset (jungles of Burma, deserts of North Africa, grassy French hills, endless Russian plains) would also be interesting. A World War I (with trenches) tileset wouldn't be so bad either although WWII battlefields are visually more interesting.
2. This tileset, thought it can only be used for indoor lvls and has only one theme, is very flexible and you can create all kinds of differet mp and sp lvls with it (but especially sp because I don't think enough good sp lvls are being made). It, being big (maybe reaching the 1.23 tile limit, forcing the author to make it TSF??), is very detailed and has a humongous amounts of tiles to use, which will encourage good lvl design/lvl originality/lvl eyecandy.
3. I know this is such an impossibly large and complicated project that nobody will bother/can make it. Three of the greatest tileset makers, Agama, Deth and Wisey, have all quit. Let's just daydream...!

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*IT IS UP TO BLURRED, ET, MIRROW, MOONBLAZE, NINJA DODO, TOXIC BUNNY AND VIOLET TO CREATE MY TILESET NOW!!!!!!!!!!!!!!!**
**SORRY FOR CAPS.***
***PLZ DON'T EDIT AWAY ALL THE SPACE IN MY POST, MODERATORS! :P