With the itemization update to now reduce the number of affixes on Rare items to 2, in order to make Rares feel special again, they could add a “rare chance” for them to spawn with a 3rd affix, and it would be an Oskill affix.
This Oskill affix would have no class restrictions, which allows characters to use Oskills even if it is not normally available to their class. They could add an Oskill branch to skill tree, and they would show up there, but the character cannot spend skill points to improve Oskills. Oskill ranks would only be improved by “+Ranks to all skills” such as Harlequin Crest.