The only input I can provide here is based on my own interpretation of the statements linked by Luna and observations about the fan Ages that have already been added to Gehn.
My interpretation of "canon" is a bit different from Luna's in that I see it as a warning against creating content that clearly contradicts existing canon (the old MOULa thread Surprise in Minkata
would be an example of this, i.e., if someone created an alternate Minkata that contained different or new content). I would expect this to include canon "physics", such as how only Yeesha and the Bahro can link to different places within the same Age. This would mean that an Age which has Books linking directly to other parts of the same Age would break the canon rule.
Aesthetically, my observation is that Gehn contains Ages which look visually as if they could have been created by Cyan. So the shapes, textures, shading, lighting, etc. of those fan Ages all are very refined. I haven't observed any requirements regarding interactivity (puzzles or no puzzles, doors to be opened/closed), Age size or environment (e.g. day/night cycle vs. static). My other observation is that all the fan Ages on Gehn are very stable and thoroughly tested; for example, they don't have visual holes (where the shapes/textures suddenly disappear) or invisible holes (where you can accidentally fall through the floor or off the side of the world); and they can tolerate having a good number of explorers milling about at the same time without getting laggy or causing crashes.
Aesthetically, I think your Lost Stonehenge Age comes the closest to the visual standard of the other fan Ages already on Gehn. I suspect the tardis would present copyright concerns, though. Personally, I don't see any canon issues with your games Ages, though the last time I visited them, they weren't as visually refined as Lost Stonehenge.
I think your maze Age (sorry, I don't recall the name ATM) also has great potential; with a bit of refinement, it could be a fun puzzle Age for Gehn. NB: It's been way too long since I've visited it, so I don't remember the "reward" at the end... there may or may not be something to adjust there, it all kinda depends.
I don't suppose any of our masters of visual quality (e.g. Tweek, Whilyam, Doobes, et al.) have created any tutorials on how others can learn to achieve similar mastery?
And hey, here's a crazy idea. How about scheduling a walk-through on Deep Island with one or two of the admins? That way, they could give you direct feedback about the good, bad and ugly. In which case, you might want to videotape everything besides keeping a chat log. Assuming the admins are available and interested, obviously.
Hope that helps a wee bit.