One possibility for the co-existence that hadn’t grabbed my attention until today has probably been obvious to everyone else: when converting from OOXML to ODF just embed OOXML-namespaced elements inside the ODF where there is no direct equivalent.
This allows good round-tripping, doesn’t require ODF to be extended with legacy Office-isms, allows developers who want to support more than the ODF base to do so, gives better fidelity for Office users, improves round-tripping and doesn’t require that competitors sit down in the same room. Furthermore, in the case of say DrawingML, the original can be preserved as well as converting it to SVG, so the chances for round-off errors and data corruption from incomplete converter implementations is lessened.
ODF already allows foreign namespace elements. I guess what ODF would need to support this well would be a mechanism to say “This kind of foreign element should be stripped out when its context changes, but round-tripped otherwise.”
The reverse is also true: where ODF supports something that OOXML does not, it can either use the customXML elements or a separate XML part.
That would be a nice use of XML namspaces, actually. Rather than harmonize into a single format, augment each other without defining new elements. I don’t know that this would be satisfactory in every case, though. The daVinci plugin, as I understand it, generated ODF where it could but resorted to nonsemantic markup (binary?) where there was elements it didn’t understand or ODF didn’t support; a better approach would be to use Office Open XML elements for that purpose.
(In a previous blog, I raised the option that a document can be ODF and OOXML at the same time. The idea here augments that considerably. But it has the same thing in common: that there are other ways of thinking about ODF and OOXML than just as arch-rivals. People think that that either ODF and Open XML will go away: I think both will be around for a while so the issue we have to face is how to manage them. Hat-tip to Patrick Durusau for the namespaces idea.)