I assume it will be Rice if they get a Senate Majority & then someone like Murphy/Power/Coons if they don't.
I actually think Murphy would be an amazing choice- he's been willing to go out against Saudi Arabia when members of the Obama team were supporting them & he'd send a bold message (while also giving himself a good step up for 2024/28)
Sen. Bob Menendez could be a good choice. He was the chairman of the Foreign Relations committee (2013-15) and has since been the ranking member. Joe Biden and John Kerry were the two previous Dem chairman of the committee. Hilary Clinton and John Kerry, the past two Dem SoS (both during Biden's time as VP), were both senators. His personal story is a plus (born to working class Cuban immigrants). If Biden wants to make Latin America a key part of his foreign policy (which I hope he does) there's no way Menendez isn't seriously considered.
Honestly there's not a chance in hell of this; Menendez opposed the two big achievements of Obama's foreign policy in both the Iran & Cuba deal.
Beside that he's a machine politician who only won in 2018 because of a favourable environment, a no-hope opponent & iirc because he was given £5 million in TV ads from the DSCC.
I'm not an expert on Latin America policy but I'm sure there's some people out there who would be much stronger than Menendez.