Whew, now that's a comment! To answer your questions/suggestions...
-Glad to hear you're enjoying the inn decorating part of the game. I'm planning to add a lot more to it in the future, so look for new decorations, furniture, and features every update!
-I'll take a look at some kind of decoration that would act as a doorway, maybe in combination with some outdoor decorations to make paths. Definitely could look cool, I think.
-I think I could make it possible to put things on the outside walls of rooms. Would need to tweak some of the placement logic in the game, but I don't think it's impossible.
-Level 2 and 3 rooms will be coming at higher inn levels, which are both bigger and have greatly expanded capacity. I also am planning to introduce some different types of rooms which will have higher capacity. Maybe something like luxury rooms, or similar.
-A second floor has been requested/suggested a lot, and all I can really say at the moment is...maybe. It's certainly not impossible, but it would take a lot of coding to make it work well. That would probably be something rolling around in the end game.
-I hadn't really considered stat boosting decorations. Could be a possibility, maybe certain items with a low range of effect, if a player wanted to make like a "cozy section" or something similar.
-Decorations not being unique is basically so players who are less inclined to spend time on the inn building section aren't overly penalized for just buying some high value decorations and dumping them haphazardly in their inn. I don't intend to make people play The Sims for hours if it's something they wouldn't enjoy. That's essentially why the guests have rent caps, to prevent this from being the end-all best strategy.
-A new house keeper is definitely in the works, but I can't say more at the moment. Look forward to it in an update or two!