ADS-B allows a plane to broadcast its GPS location to other planes and ground stations used by ATC.
The range is 200nm for A/A and 180nm for A/G (source).
Same SSR coverage issues would still impact oceanic areas.
But what if a train of planes (crossing the Atlantic for example) communicated what they see to each other, until the one closest to a ground station relayed all that information to an ATC facility? Is it being planned as a future update?