I doubt making every LW account a valid PB account would do the job alone, although it's a good start.

The main directions here are 1) to drive traffic to PB, and 2) to identify and remove the biggest trivial inconveniences. Having separate accounts is definitely near the top of the list.

X most recent predictions, similar to OB and the LW wiki.

Yes, this is exactly what I had in mind -- a sidebar thingy (is 'extension' the official term?) that serves three key functions:

1. Gives new LW visitors an idea of what PB is.
2. Reminds existing LW users that PB exists.
3. Drives traffic to PB.