I think in theory this would be an awesome idea, but from what I've seen over the years community members aka devout fans of Ultima Online really don't appreciate or want change. They enjoy their classic clients and their own ability to alter the world in 2D isometrics; the way Ultima Online was really meant to be.
Many have tried to create custom clients in the past and only Kirros was successful with it; but he's never released the source code to my knowledge. Iris2.de had some success with their custom 3D client they made using Ogre but it soon failed because of graphical glitches. Florescence was developed by a guy named Spin... have no idea if he's even come close to finishing his project. It was a 2D version of the classic client but had cool effects built in like animated wavy water. Finally there was the XNA Client and like the rest it went by way of the dodo bird I think. All these wonderful ideas started by people who meant well, but never had the time to finish or just got burned out because of stress.
If you plan on building a client.exe you need to know that people will be hounding you, until you break, to finish it. That doesn't mean everyone will, but there will be a select few who are impatient and criticize everything you'll accomplish. Your best bet is to create the client.exe in private... don't tell anyone and when its done, release it if that is your plan. It will save you a lot of headache because not everyone on here is polite.