You can do a lot of this with sub-projects, the "media agnositic" tracks within SE plus keyframable clip properties. I found that sub-projects can be CTRL-dragged to a Filebin and later used just like a clip in the same project in which it was created, or in a totally different project. (The sub-project shows up on disk as a project file. Its icon display in the Filebin at this point is not reflective of the comp but of one track in the comp - probably a simple fix.) About the only function I can think of that is missing are complex mask generation, and perhaps a way of merging or grouping a mask track and its effected clip track.
I recently duplicated a project in SpeedEDIT that had originally been done usin Avid Media Composer and Boris Red. In SE I was able to build my comps without leaving tghe SE environment due to the positioning, cropping and property key-framing available within SE itself. So it seems to me that, unless it is a really complex comp, SE can handle it already. Anything too complex for SE should probably be done in DFX+, AE or Boris anyway, beccause it probably requires tools that you don't want to burden SE with at the expense of SE's leaness and meaness.