An online miniatures map would be helpful, especially for 3e and 4e where miniatures are more important than previous D&D editions.
Adobe Flex and Papervision3D would be initial thought on what I'd like to implement. 3D is good for future-proofing as well as making a nicer table view.
A few requirements might be:
One option is to just take an existing Flex/Flash/AJAX/DHTML open source miniatures map and repurpose it for ajaximrpg.
Another option is to take an existing Java or Python open source miniatures map or application, translate it to Flex and use it for ajaximrpg.
A third option is to do a Flex implementation from scratch although that'd be very time consuming. A series of phases would be most appropriate; get something basic out quickly and then iterate and improve that with new releases. I'd have to be careful that the miniatures map doesn't eat all time and resources, though.