Adding damage, adding health, speed etc, just makes Nightsisters better, but its very generic. What most factions need is someone to bring them together, and empower them beyond just health/damage. Do X, get Y and Z.
Ackbar does this for rebels, Palp does this for Empire, HK does this for droids. These are team altering leader abilities that have strong synergy with the specific factions. Nightsisters need something to that effect that allows them to all work together and to promote stacking a team with them.