不然 when it occurs at the beginning of a sentence
不然 means otherwise in a declarative sentence and alternatively; how about in an imperative sentence.
In spoken language it can begin a sentence to propose 'plan B', which implies a self-evident 'plan A' that is favorable to one party but not the other, and there is no need to talk about it so the speaker jumps to the alternative plan already.

不然你逛那边的时候，我逛电子商品区.
(Hidden context: ideally we should stick together but we couldn't for some obvious reason, like one person is not interested in the stuff the other person wants to shop, or both of us need to buy something but we're running out of time.) How about this, I am going to look around for the electronic products while you're shopping in that area.