I think the biggest thing to think about is which option would allow your casual map mapker to come along and fiddle around with the new features you are going to implement, without adding a serious learning curve? 'Cause I remember trying out Photoshop...hmm..all my attempted textures ended up looking like gradients.....
#1 sounds great, and who cares if it's not "proper", so long as it works. But I'm not quite sure what you mean by "the proper way"---"more standard way" of doing things.
#3 seems fun, I think I've seen this in UT before (feel free to correct me on that if I'm wrong). I think the effect was something like The Never Ending Stairs. Jump down one floor, walk around the corner, boom, you're back at the top of the stairs you just jumped from. But you say it'll take a lot of work, so unless many vote for it, I say bah to this.
Concerning #4, would manually creating floors/walls/ceiling be done a 3D modeling program or something? If so, and you chose to go with this option, then maybe your "best of both worlds" idea would indeed be the best.
There's my opinions. I hope I didn't paint a bull's eye on myself by saying something stupid up there. Off I go, back to my dark corner.
