As far as I've seen all notifications are kinda the same. They just contain a flag which states it's for SMS, e-mail etc. So after "converting" all notifications to the new protocol for the timeline all of those should work. But that of course means just the notifications work and are put on the timeline. That doesn't mean you magically get full support for the timeline, which can also be filled by remote services and apps. So the first goal would be to convert all the existing notifications to "pins" on the timeline. And when those are working it should be possible to investigate how to add support for external/web services[1]. And then it might also be feasible to add some kind of API for "local" apps to add pins to the timeline.

[1] From what I've read in the official documentation the timeline is foremost filled based on external services. Using the Pebble app you have to subscribe to a service, after which your subscription will be registered by Pebble (the company/website). The external service then has to push the pins to Pebble (company/website), after which the Pebble app will then query Pebble (company/website) for all new/updated pins and push those to the watch itself.
So luckily Pebble took some effort to make it possible to fill the timeline for all kinds of mobile devices without requiring native apps. So when this is working (which will take some time, to investigate how the API between the Pebble app and Pebble servers works) a lot of the shiny timeline data might become available. Like all those sport based pins.

Edit:
Forgot to add that filling the timeline based on the phones calender would of course also be feasible. So that should be something to add after (re)adding support for sending notifications.