the Dev Diary shows that the Hive Mind Ethos takes 3 'points':
but the Hive Mind being an ethos made me think that maybe we could get a deeper variety of them: there would be a scale of 4 "Mental Structure" Ethoses that you would have to mandatorally pick one of to determine what kind of species you are:
Individuals: basic vanilla Stellaris; each POP as it's own happiness and Ethos. Costing 0 'points'
Biological Caste System: the 'social' hierarchy of this species is written into their genes, each has their place and it would be foolish to say otherwise; POPs have their own happiness and Ethos, but can only have caste system' citizen rights and will always have 100%(+200% for the trait bonus?) tolerance of slavery for their own species regardless of their Ethos. Costs 1 'point'
Melded Society: each "individual" is technically an independent entity, but can commune directly mind to mind with others of its species; POPs have their own Ethos, but will share a collective happiness level on a per Planet level, and will suffer a large penalty if there is not a large number of same species POPs on the planet, they are ruled by a collective gestalt personality that provides an additional happiness boost to all living under it's rule(but those outside can live without it), and will change it's "government Ethos" based on whatever is most common amongst it's POPs. Costs 2 'points'
Hive Mind: a single undying collective consciousness controls the entire species. works as described in the Dev Diary. Costs 3 'points'
each of these gives the species a trait of the same name that can be gene modded into the others later if desired.
and on the subject of gene Modding and Hive Minds: I think the Hive Mind should start with a trait like "Natural Hive Mind" or "Biological Hive Mind" and only be able to control it's own species, not being able to rule other Hive Mind species until Gene modding in a "Expanded Hive Mind" or "Artificial Hive Mind" trait into itself, and even then they'd then need to add a "Hive Mind compatible" trait into other species to control them directly: this trait also allows the POP that has it to survive outside Hive Mind space as this trait would be in addition to rather then replacing their Mental Structure trait(allowing you to "save" or "liberate" those who have been wrongly "subjugated" by it)