While still very early on, mock up shots of a Dinosaur based combiner, loosely resembling the Dinobots (or at least their animal-modes), is in the works from TFC Toys.
Check out images of the proposed Combinosaur after the break.
UPDATE: More pictures have come to light of the individual dinos, all from ACToys. There are CAD designs for the T-Rex, Triceratops and Pterodactyl, plus finished renders of the T-Rex. Click here and here for the original sources. 2005 Boards member ki1983ng informs me that these are indeed intended to be third party Dinobots who can combine to form The Beast, but everything is currently still in the planning phase.