I think they should put Ingress' field game play back in. If you have a Psychic type pokemon defending two gyms, then you can link those two gyms up. When 3 links forms a field, players from other team under the same field will experience a higher rate of pokemon escaping on them.
My original thought is to have the other teams' tracker disabled, but since everyone's using Go Radar to locate rare pokemon anyway, it might be a moot game mechanics. To have them unable to capture pokemon seems severe. So maybe a higher escape rate would be enough.
That might prompt a higher level of team play. As teams have to consider how to stack up their gyms and place a high CP psychic type, which is pretty useless right now, on the gym to maintain the field as long as possible.