Add three unassigned Draft BIPs for the timestamp-boundary fix, the legacy sigops transaction limit, and coinbase locktime duplicate prevention. Attribute the new drafts to Jeremy Rubin, leave BIP54 and BIP53 unchanged, and copy each new draft's test vectors into its own auxiliary directory.
As per discussion on #2175 (comment) this seems to be the favored approach. @Christewart, I think it's worthwhile (separate from this) to make BIP-0053 have the same test vectors and spec languages as BIP-0054 for the 64 byte rule.