As discussed, the obvious first thing is to make a DF600 that sits between the DF500 and DF700. I once posted that this would be the equivalent of making a ⅜" socket set. Yes, you can cover everything with a ¼" socket set and a ½" socket set, but you'll find that 90% of the time the ⅜" covers what you need and is just the right ratchet size in your hand.
My personal recommendation for sizing:
Bit Diameters: 5, 6, 8, 10, 12mm (lose the 4mm and 14mm sizes) Push comes to shove, lose the 5mm too.
Dit Depths: 12mm - 50mm. The deeper option isn't just for bigger joints, it's also to enable using dominos for real strength on cope and stick joinery in ¾" stock. Festool will have to work out how to lock out the deepest depths for the 5mm & 6mm bits.
Festool made a number of improvements in the DF700, which came out after the DF500, that I'd like to see also incorporated in a DF600:
• D-Handle
• Fence design of the DF700 is overall better.
• Fence height markings should be like the DF700 - Actual distance from cutter centerline to fence, not the DF500's stock thickness markings, which screw you up if you're not putting mortises in the center of the stock.
• Note also that the DF700's use of a step wedge for height stops enables aftermarket/self-made 3D printed replacements for things like custom offsets or imperial offsets. This is cool.
• DF700 depth adjustments include a Minimum and a Maximum Limit. This is really handy for right angle joints where you can't go as deep into the face as into the edge, or just for offset dominos in special purpose joints (like chair joints).
• The stop pin setup on the DF700 has more options than the DF500.
I'm torn on having the 3 slot widths - I don't miss that on my DF700, but others might.
Finally, as someone suggested, while there have been some really clever mounting jigs for Dominos (ranging Ramon's vertical Dock to the inverted table mounting that Woodpecker's copied), it would be good for Festool to think about angled joinery and small piece joinery and come up with something better than the Narrow Frame Fence thingie.