Designing a sound that localises well
You want something that has lots of features in the spectral domain and in the time domain. However you have to be careful what other content or meaning you would like to convey or not. Spectra conveys pitch and/or coloration and timbre. Time transients convey rhythms and/or patterns.

For example you could do band limited noise burst with different center frequencies and patterns. That would allow yo to vary localization, spectrum and rhythm without directly conveying pitch. You could also use different percussion samples with different rhythms.