The community's sort of made it a point to avoid labeling any fan-made campaigns as "canon," even in the informal sense; the general consensus seems to be that designating one campaign as such would belittle the work of other campaigns/mods set around the same time period. That being said, there are a number of post-FS2 campaigns that are widely considered to have a very high standard of quality and fit into the general scope of what a sequel would cover; some of the more prominent examples are Blue Planet, Derelict, and The Procyon Insurgency. There are a number of other projects that have been in development for some time that will also wind up covering much of the same time period, including Blackwater Operations, Machina Terra, Twisted Infinities, Earth Defense, and the revamped Inferno. As far as stuff that isn't set after FS2, Silent Threat: Reborn is an excellent reworking of the rather-underwhelming FS1 expansion campaign, the Ancient-Shivan War project is tackling the Ancients' original war with the Shivans, and the Terran-Vasudan War Project will eventually cover the time period immediately preceding FS1.
As for plot overviews, the
FS Wiki has a lot of information that would probably do the job; the FS1 and FS2 sections have full campaign briefing texts for each of the retail campaigns, as well as mission walkthroughs. Most of the fan-made projects have their own wiki pages and official websites, too. If you need more detailed info beyond that, I'm sure many people here would be willing to write a brief summary up.
