An “iconStyleClass” attribute could be useful on tree nodes in order to use Bootstrap-style class-based icons - a renderer could look for that and, if no “image” property is present, use that to generate the element. Currently, I'm piggybacking on “imageAlt” for that, which is okay, but not ideal.
More generally, having an “attrs” property for arbitrary attributes would be useful. In my case, I want to include a numeric badge in the fashion of http://198.74.61.72/themes/preview/ace/ and don't want to cram it into the label (since it expects to be HTML-escaped). Currently, I'm piggybacking on “title” for this, which leads to an undesirable tooltip.