Ideally it should be some meeting point over TOR so that there is no incentive to try to record IPs.  Though I'd prefer instead of opportunistically swapping that it rather had lots of people indicate an intent to swap, and then when you want to make a transaction, you'd jointly create a swap and pay transaction. This avoids bloating the blockchain with a bunch of pure swapping and would further improve privacy as you wouldn't know _which_ outputs were swapping and which were payments.  Payments to common anonymous donation addresses could even be merged.