Lead by example
One thing we tried on the (now discontinued) Startups SE was to try to lead by example.
We'd set a theme on meta, and then invite regular users to post questions on that topic. The moderators picked themes from a meta question to coordinate the whole thing. The experiment was fairly successful insofar as I can recollect.
One mistake the mods did was to periodically pick cringeworthy themes. These led to obviously forced questions that attracted very low quality answers. Another mistake was to cycle through themes too quickly. It was one theme per week or fortnight (can't remember); a better rhythm might have been a month or two. These mistakes seem easy enough to avoid if we try it on History. The benefit besides a regular stream of good questions it generated is that newcomers had questions that met expectations all over the site to read before asking.
The main issue I'd be concerned about if we were to try this for History is whether we're able to find a large enough pool of non-trivially searchable questions. ;-)