As we pretty much have to pick 1) gobgob to even summon, 2) posession- to have little control over our summon and make it use some spells they wont use otherwise 3)osmodas blessing- you can argue that but I think its a must with current control system 4) Gobup- needs no explanation
that leaves room for 1 free spot and theres not really much to choose.
If you take all 5 current active specialties that allows you 6 additional spells which is technically the same as a current Tribrid. You just won't get any utility spells.
THAT SAID, Osa already suffers from a problem of 'one role'. If you embrace that, you actually won't need to take most actives into most fights.
If you don't understand it you should make one of the 6 classes that works on Beta. Sram, Iop, Cra, Sadida, Enu, ect.