I'm in the frustrating position of being at 2/3 done and I can't get anyone for my last stunt mission. This could all be easily fixed by adding a "Looking For Group" Interface so that we could look for other people who want to do the same kind of challenges we do, and even better, an option to look for others who want to do a specific challenge. Searching for players should also give us the ability to filter out people who are already in full groups, and if players are already in a challenge and allow us to join the group, we should be able to join that challenge immediately instead of forcing them to restart it for us.
These are easy work arounds that have been around FOREVER in online games, so please get them pushed through in a hurry. Seriously damages the experience to not have these simple QoL features.