as a user i would like to link issues with custom relation types over the standards "linked to, dupplicated, ecc.."
like "describes, implements, ecc..." with no business logic rules implemented. Maybe it would be useful to permit to select the already implemented ones (blocks, blocked, duplicate).

and it would be very useful to lock the custom link filtered by tracker, for example: the new link type "implements" is available only for the tracker "feature" and can link only issues from the tracker "business request".
It can be obviosly extend to multiple tracker selection and not only one.