Possibly. Most games these days do not get more than one Expansion Pass, and I would assume that the remaining 300-ish Pokemon would have to come with more DLC. I think it unlikely Game Freak will want to make more DLC for SnS next year (after all, all the content they cut from SnS will be in the existing pass, so they'd have to actually do something new) and would instead be focusing on the next games/oft-coveted DP remakes, but if SnS continue to sell well this year and the Expansion Pass is received positively by critics...well, it's possible. But if we're assuming no more DLC, I would say it's extremely unlikely they'll just patch the last ones in. Too much effort, not enough profit.
So, who knows. If they did it'd certainly be highly entertaining, after the Dexit debacle and their stubborn insistence that this is how it's going to be from now on. If they don't...well, not much changes from right now. An additional 200 still isn't the full dex, so it's really not that impressive, and certainly not something to be grateful for.