I think it would be terrific to to combine/have one but is this extra work? Usually we just create a single one each year.... Whatever you think would work best. We need to get this done asap so the folks can announce/put up the conference wiki in the next couple of weeks for 2018.... Thanks, ellie
Given this, and no clear ownership for making the decision for a combined wiki, let's simply create wikimania2018.wm.o for now and *immediately* file a task about a combined wiki moving forward that we can bike-shed on for at most 11 months (before the call for wikimania2019.wm.o is filed).
Discussion for one consolidated wiki
There are ongoing concerns to consolidate all Wikimania wikis into one, with a suggestion given by @Addshore to use namespace per year.
@faidon expressed concerns about search, addressed with the suggestion to configure search to be tied to the current edition by default.
That would have benefits for contributors: templates would be there and available, everything would already be configured.
Proposal to move forward
It could be interesting to create this wiki as wikimaniawiki, and experiment this unique wiki for the 2018 and 2019 editions, with the "one year, one namespace" pattern and see if it works well.
After the 2019, we'll be able to see if it works well. If so, we'll use this wiki for 2020+ editions too, if not we'll create wikimania2020wiki.