you really need to define your definitions, or at least understand the ones you have given
a waveguide opening or a horn antenna/radiator is, by definition, directive. They are NOT omni-directional. A slotted waveguide radiator (as suggested ny one other poster) has an excellent omni-directional radiation pattern which is made even better by the use of "wings/fins".
do some goodle searching on slotted antennas or read up some of the ARRL antenna handbooks
Dave