Regular expressions and Visual Block Selecting are features I wouldn't be able to live without.
An optional feature would be workspace support, that is when https://github.com/nex3/perspective-el comes to be a very handy tool.
While both visual block selecting and regular expressions are key features of emacs that we rely on day in and day out (either interactively or by way of stock functions,) I'm not sure that they're the kinds of features features that make sense on a tablet interface.
Perhaps I'm using the wrong input methods, and perhaps I have everything backwards, but most of the time that I'm using the tablet are when I'm waiting for a train or on a train, and sometimes when I'm hanging out on the couch. That's not the kinds of time that I want to be constructing regexes on an on-screen keyboard.
I think android and iOS are on the verge of bursting out of the tablet-form (there'll be more keyboards in time, as well as Android runtimes for other platforms,) but I think for now the tablet use case is really important to keep in mind.