Currently, in Bitbucket, you can reference other issues using the #\d+ notation, for example: BCLOUD-8051, BCLOUD-5633 (arbitrary issues).

One interesting feature could be linking issues in other tracker by prepending the repository name to the notation: tortoisehg/thg/#470. Which in this case, would become something like:
tortoisehg/thg/#470. With a dash if it was closed.

An interesting complement to this, is that, an issue created inside a user/team, would preferentially reference a repository hosted by the same user/team. thg/#470 on the "site" team, would reference site/thg/#470 instead of referencing the one in the previous example.