This may be overly ambitious (if that's a term we use around here), but what about reserving '.' as a subdomain delimiter, and '/' as a protocol and path delimiter, but allowing any other character sequence as a domain, no designated TLD required. We could at least temporarily reserve/block all existing or already reserved TLDs until they can be ported to the BitShares DNS system, and the initial plugin would just propagate requests to those TLDs into the traditional DNS system.
To minimize loss of stake as a barrier to entry, it would be good if there were a way to seed the system with a snapshot of current domain ownership, but I don't yet see a good way to do this.