You don’t want an omnidirectional speaker. Your dipoles are as good as you’re going to get for both imaging and spaciousness (artificial) without cabinet coloration.
Otherwise, look for speakers with incredibly well damped cabinets, controlled dispersion and steep crossover slopes (perhaps active crossovers for best results).
Driver quality also plays a large role. The higher the distortion of the drivers, the more they smear the image IME. Same for crossover slopes—low order/shallow slopes cause the imaging to fall apart when the music gets complex or played loud.