Un-similarity would generally just mean further away in the latent space. I don't know if there are common heuristics used for this - taking the furthest away w

Un-similarity would generally just mean further away in the latent space. I don't know if there are common heuristics used for this - taking the furthest away would give you outliers, and then probably lots of members of whatever class was furthest away. So you'd probably be best first clustering in the latent space and then sampling from each cluster.