Example possible BIPs:
URL syntax for bitcoin payments
New OP_EVAL scripting opcode for receiver-specifies-transaction
New bitcoin address format to support OP_EVAL
(I'm actually working on those two)
Process for announcing/scheduling/implementing a potentially block-chain-splitting change.
You can browse through the PEP's at http://www.python.org/dev/peps/
to get some idea of the kinds of things that might be good BIPs; changes to the on-the-wire protocol to make blockchain downloads faster would certainly be appropriate.