If you want plugins in the current manner you're used to them (i.e. round tripping), you'll likely be waiting for quite a long time. The new approach is via extensions which allows any application to add to the render chain to make non-destructive changes to the image. This has been opened up currently on iOS Photos but not yet on OS X. Now, whether it will take 5 years, 5 months or 5 weeks, I can't say .
It's just not that kind of application.
Think about it in context with the numero uno feature: iCloud Photo Library and how changes on one are reflected on all your devices. Probably it's much more likely you'd see an app on iOS that could apply Instagram-like filters. I wouldn't be surprised if more Photos users by far will edit on their phones, where they took the picture, than they ever will on a Mac. Just look at the difference in the number of desktop vs mobile applications.
No company that lets a piece of "pro" software go as fallow as Apple did Aperture was aiming to replace it. There was a skew in hopefulness here and elsewhere because lots of people left Aperture when it was obvious it was being neglected. So the only ones left were, um, overly hopeful shall we say.