It would be difficult, but not impossible to reform Team Rocket in Kanto. They just need to hide out until they can get enough people to overpower the police, or do such a big strike against Kanto to leave their forces crippled. It would be difficult, but it wouldn't be impossible. It has been shown that the defences were upped in the original G/S in case of Team Rocket's return, but it isn't impossible to reform there. Besides, there could have been many people secretly working for Team Rocket in high positions that weren't caught... yes, probably not true, but it is possible. Also, one of the underground tunnels was closed, and Cinnabar is almost deserted, there could be networks in those two places that no one knows about, and if they decide to add it in, than the Sevii Islands. They didn't do anything major there, just looked through two places for two stones, nothing drastic enough to really get worried, unless they get paranoid of course.
Team Rocket showed potential to return in G/S. Maybe it doesn't mean anything, but there was a member in Kanto who took a part of a machine from the Power Plant, which, since it is possibly the biggest power plant in Kanto (If there are more, but realistically speaking, there should be more.) It should have some sort of security, because if it were attacked, than the consequences would be devastating. It supplied power to the Magnet Train, and possibly supplied thousands of buildings, including major buildings with power, but that was unable to be touched on, as that would cause the region to be shut down.
But I doubt they would go through the hard work involved to return to Kanto... maybe Sevii Islands, but not Kanto. Redstar's argument makes a lot of sense, more than mine in my own opinion.