Your comments

I would suggest a popover, or, if it's not too heavy to implement, a "Mover"-Button, on klick, the sidebar fades in or out.
An outline key set would be great!

Also multiple, user-defined, key sets. But I, personally, like the tab key as the left-most key, just like on a regular keyboard. There are not many apps in the AppStore, which have this, or only a few I've found. So this is great.

I would suggest not to build some already predefined key sets, just give the user the ability to build their own. Every user has its own preferred keys, he wants to use.
I would prefer a mixture of 1) and 2)
- I can add and edit files/folders local and upload them to my Dropbox.
- The other way, I can download files/folders to Textastic and have a local copy for offline editing.
   But if there is a network connection available, they should get synced.
- Support for files _and_ folders would be nice - normally you just edit one file, like som notes or a letter,
   but if I want to have e.g. a whole XCode project or a Website project in Textastic, you don't want to sync every
   file manually, instead sync the whole folder.

3) is nice, but iPad is designed for mobility, and if you don't own an iPad 3G and don't have WiFI around, you're not able to edit your files.

If Dropbox is implemented, please don't set a fix base directory, like /Textastic :-/
I like my Dropbox organized and there are lots of Dropbox tools around, which set their own base directory. After some time your Dropbox is messed up with many different directories. I, as a user, want to decide on my own, where to store my files.

Anyway, awesome work on Textastic!!! :-)