Resubmitting book proposal?
Would it be allowable (or prudent) to resubmit a book proposal


Usually, Yes, provided the revisions are substantial and have materially changed the book for the better. 

You will need to read the proposal submission terms if the specific press allows resubmission of a rejected book. If they expressively and univocally prohibit it, don't bother. 


  If so, what elements should be different or included in the new proposal


Please read the initial rejection letter and see if the reviewers have left any criticism/reason for turning your book down and see if the reasons cited have been addressed in your recent revision. 

If yes, do highlight these revisions and include them in the new proposal/covering letter.