What -- no takers?
This is the most brilliant idea that's been posted here in weeks.
Where is everybody?

The idea has been following me around all day.
Developing... The possibilities go on and on.

Why do the dementors like Harry so much?
Why did they search his compartment on the train?
Why did they single him out at that Quiddich match?
Why did they attack him before attacking Sirius?
They were looking for a murderer. Can they sense a damaged soul?
If Harry is an accidental horcrux, does he somehow stand out to them?

How do they communicate? We know that Fudge talks to them.
They told him that Sirius had been talking in his sleep.
And that they were "angrier than I had ever seen them."