Of course you can! Though I don't know how much billboard space I have left, post all your ideas and I'll see what I can do.

Well, while birds flying by in the background is impossible (All bg layers are used up), you might see birds flying in the foreground at night, in the morning, or when raining (that's when there aren't cars on the street and I have free foreground layers). Thanks for the idea!

I have something similar to this planned already. I'm going to include in the upload to J2O some music tracks which are going to be used for individual buildings. That way, when people join the dedicated 24/7 server, when they enter a building, an invisible /changemusic command is sent which changes the sound track.

Of course, that's going to be possible if you want to host yourself. However, if you choose to do so, you need to have a console program running in the background (that only you, the host, need to run, not the clients). That console also adds things such as global trigger zones (which are mandatory) and other miscellaneous features, so it's necessary for the host to run it when playing Carrotopolis. Don't worry, it has no buttons, so you can't break anything.

Also, if people don't download the music from j2o and the player attempts to changemusic, then the music simply doesn't change (no crashes and the likes). Of course, that's pretty limiting to the ambience and overall experience, so it's certainly preferred you download the level (&music) from J2O so you don't miss out on anything and don't waste time downloading on jj2.

I'll completely redo the first post later today, so look forward to it!