This time next year, I'll probably be living with you.
Literal
Next-year [possessive-の] this-time-around [topic-は], you [with-と] together live-is [probably-だろう].
一緒に住んでいる is 'be living with / cohabit.' The ~ているだろう stacks ongoing state + probability — 'will probably be [in the state of] living with you.' A romantic, plan-ahead sentiment.