To present a complete new Expansion for PC needs again also a lot of promotion. So I think, that a first Expansion Pack, Add-On, DLC or however they will call it, will be shown around middle of August 2017, when the Gamescom in Cologne starts. That's about 4 months, nearly the same time between the last two big (free) updates from December last year to April 2017. If they just can show us then only teasers (or spoilers), the next part of PC could be then available for the EGX in September. I think this both events will be regular events in Frontier's schedule.
Or they release the next official Pack in November like they did with the Final Release last year. Better start for christmas sellings.