Len has given me an idea...
Are you me?
My first is unpredictable, if you add some from the middle
My first is rand; adding the middle half (om) of "some" gives random - a synonym of unpredictable.
My second is one, but a short line splits it apart
My second is al'thor: one word, with an apostrophe (short line) splitting it apart.
The first part is a common word, if you come from the Middle
The first part of al'thor is al, which is the translation into Arabic (a common language in the Middle East) of the commonest word in English. See here.
The last is worthy, coming from the bottom guard
The last part is thor; Thor comes from Asgard (which is a homophone of arse guard, which is a synonym of bottom guard), and he's the only one 'worthy' to lift Mjolnir in the Marvel films.
almost everyone here has seen the answer.
I'm very active on Puzzling.SE!
I'm very flattered by this riddle - thanks CodeNewbie for the compliment! :-D