A great mod indeed !
Phpbb lacks a good userCP and this mod could really improve phpbb in that area that i think will only be improved in phpbb 2.2.x
That's the features i would like to have in a usercp:
my last X posts
my watched topics (just a list of the watched topics that already exist in phpbb)
But the main feature i would like UserCP to have is modularity... i mean the user can configure what "modules" would like to have in their usercp: if they want to have my watched topics and my last x posts or maybe only have my last X posts, etc... i think you understand the idea... Maybe this is more work for you so maybe do this only on 1.1.x of the mod?
Also, this userCP could be a great place to integrate other mods so... keep up the work