Often the key to success is to not try to do too much. That was one of the big reasons the PDA market was dominated by Palm Pilot.
Earlier PDAs like the Sharp Wizard and the Casio BOSS tried to do too much. They wanted you to be able to fully maintain your calendar and contacts and todo lists and notes on them. But with their little keyboards, slow processors, and limited displays they weren't very good at doing a lot of data entry or editing related to those tasks.
Palm viewed the PDA not as the place all that stuff would live. In their view all that stuff lived on your computer. The PDA was for taking your data along when you were away from the computer. You might take notes on the PDA but then when back home you'd use the computer if you needed to do much editing of them. You might add or delete a todo item here or there, but if you were going to extensively reorganize your todo list, you'd do it on the computer.
I think Apple is, correctly, thinking that the Apple Watch will do best if it is like the Palm Pilot. It is a peripheral for your iPhone and doesn't try to shoehorn into the inherently limited interface support for doing things that can be easily done on the iPhone it is paired with.
If they tried to make it standalone without greatly reducing the functionality they would just end up with something very complicate to use.
What they could do, without making it too complicated, is make it so you can pair it with things other than iPhone. iPad and Mac at the least. Maybe Android phones/tablets and Windows PCs too. But it would still be primarily a peripheral for those devices, not a standalone device.