We are aware that there is the occasional problem with people being able to start their Legacy missions after defeating the last Plague Heart on the map. You will know you have defeated the blood plague in your zone when your Primary Objective changes to Promote a Leader or Define your Legacy. It is also communicated that all Plague Hearts are defeated through character dialogue. The character will say something about this Plague Heart being the last one. There is also generally some chatter about defeating the blood plague after its defeat.
If you are certain you have defeated the blood plague, then we are aware of a couple of situations in which players can be blocked from starting their Legacy mission.
The first scenario is that there is an active Leader mission (or sometimes a mission for another community member's personal arc) for someone in the community. Start that mission - you can complete it or abandon it - and that should get you started.
The second scenario is not as well defined. That said, it seems that eliminating some enclaves from the map will usually get the Legacy missions started.
If neither of these things starts your legacy, please submit a request and we will try to gather more information.