RE: tx replacement:
Somebody aught to start experimenting with transaction replacement with testnet-in-a-box.
Put together a test plan, modify the code and try to break it.
Assuming it all works, submit a pull request to enable transaction replacement on testnet (with a pointer to your test plan and results of testing, please).
Then we can start trying to break transaction replacement on testnet; again, assuming no bugs/hacks/issues turn up after "enough" testing, and if there's general consensus that the benefits outweigh the risks, it could be rolled out to the production network.
We just pulled a DoS fix involving filling up memory with orphan transactions; one concern with transaction replacement is DoS (could I fill up the memory pool by spamming "will be replaced" transactions?)