Hallo
I would find quite useful some option to change tempo between two tempo marks continuously. At the first mark option window would be an chack-box “change tempo to next mark continuously”, or somethink like that, as it is e.g. in Rosegarden. (In addition, there could be an option for linear, exponential etc. continuance.)
Similar ideas have already popped up
http://ardour.org/node/256
http://ardour.org/node/251
http://tracker.ardour.org/view.php?id=1224
http://ardour.org/node/396
Personally i really liked the tempo track system of cubase.