With Rekonq being a relatively minor browser, it probably makes not much sense to develop a unique extension API. Instead Rekonq should implement existing cross-browser APIs.
This feature is probably the best candidate to be implemented first.
Right now, Mozilla Firefox extensions require XUL support to run. This makes it very difficult to implement in non-Gecko browsers.
While the final JetPack specification will be released with Firefox 4.0 (currently targeted to be released in Q4 2010) prereleases are available right now that should at least give an overview which preconditions are required to be compatible with those extensions and whether it's even feasible to implement that API.
If not too complicated, maybe even compatibility with JetPack and Chrome is possible.