next in line
1. Literally, next to be helped or served. I'll help whoever's next in line. Excuse me, but I'm next in line. You'll have to wait your turn.
2. Figuratively, next in the order of succession to a position of power, immediately after the current position holder. Rumor has it that Margaret is next in line to take over the company after the CEO retires. The birth of the young prince means his uncle is no longer next in line to the throne.