[toriningen]

Or you could have CQRS projectors (read models), which solve exactly this - they aggregate data from lots of different eventually consistent sources, providing you with locally consistent view only of events you might be interested in.

It will lag behind by some extent, roughly equal to the processing delay + double network delay, but can include arbitrary things that are part of your event model.

Though, it's not a silver bullet (distributed constraints are pain in the ass yet), and if system wasn't designed as DDD/CQRS system from the ground up, it would be hard to migrate it, especially because you can't make small steps toward it.