some, one, or, possibly, a certain
或 centers on indefinite reference: a certain unspecified person or thing, some one among possibilities, or the logical alternative 'or'. The meanings 'possibly' and 'a certain' are natural extensions of this idea of uncertainty or selection.
或 originally depicted a territory or region, combining 戈 (halberd) with 口 (enclosure), and later came to be used for the indefinite pronoun 'some' or 'or'. The modern form retains these components.
Imagine a halberd 戈 guarding an enclosed area 口, but you're not sure which area—it could be some place, or possibly a certain one. The uncertainty of the guarded spot gives 或 its sense of 'some' or 'or'.
For ワク, picture a vague, wacky choice: you have to pick some option, but they all seem wacky. Wacky -> ワク, and the kanji is all about uncertain selection.