Arguably, Drupal is an excellent CMS; it’s used by “hundreds of thousands of projects” (from professional to proprietary to educational); it’s the platform I selected for my most recent DH project; and it’s one I’d recommend to others (it’s learnable; teachable; extensible). But I would by no means necessarily advocate Drupal over any other CMS.
The way I understand the future of TEI is by learning from the past, or specifically my past. I will let someone more worthy provide a more generalised set of proposals, I will look at the different ways I have used TEI encoded documents, which I hope will show the trajectory of my thoughts about how TEI could be used in the future.