While we can't say what will come from the epic finale, we do know that production ended in Aug. 2018.
We're going to assume there will be a huge showdown with Jonathan, who right now is trying to lure Clary to the dark side.
If you remember the shot of her soulless eyes in the Season 3B trailer, we can assume he might have been successful.
The epic 2.5-hour event will take place on May 6!