The concept of switching the enemies' gravity is interesting, but in the implementation, it didn't seem to have much of an effect; whether they were on the ground or the ceiling didn't make much difference in terms of whether I could shoot them. Perhaps some designs in the levels could have fed into that mechanic a bit (e.g. spikes on the ceiling for an enemy that can't be killed by shooting).
Also, having the enemies do things might have been interesting as well. Maybe _they_ can switch _your_ gravity. Or at least shoot at you.