Shohei Ohtani
Yeah, you probably knew this one was coming, as Shohei Ohtani is going to be the most watched name on the trade market this year, given that he is about to hit free agency and make a HUGE sum of money that could be in excess of $600 million. It is not hyperbole to say that there is not a player like him on the market (as in, one player). He would instantly become most teams' best hitter and best pitcher in one roster spot. He is doing things that we haven't seen since before World War II and it is beautiful.
We'll start with the reasons why this could happen. First, he is incredibly good, as he has slashed .294/.362/.515 this season with seven homers, while also posting a 2.74 ERA with 66 strikeouts in 46 innings of work on the mound. He has won a Rookie of the Year and an MVP award already, and had a really good case to win AL MVP last year, when he finished second. Also, he is on an Angels team that is currently kinda competing, but has a poor track record of actually finishing a complete season well and stands to lose Ohtani for just a draft pick if they don't trade him. Finally, the Dodgers have long been rumored to sign Ohtani and it sounds like the feeling is very mutual, so there would be an opportunity for LA to get an exclusive extension negotiation window before the season ends (and before other bidders can enter the fray).
There are some issues here, though. First, even though the trade would technically be for a rental since Ohtani's contract is up at the end of the season, he is still going to be very expensive in trade capital. Again, there isn't another player out there that gives you the sort of 2 for 1 value that he does, even if it is only for half a season's worth of value.
More importantly, the Dodgers seem more likely to try to get under the luxury tax by the end of the season to reset their penalties so that signing Ohtani in the offseason is more feasible/less painful. They could, in theory, shed a big chunk of change ahead of time via other trades to make room for the remainder of Ohtani's contract in 2023 and still make that plan work, but that could be easier said than done, unless the Dodgers part with some good players to make it worthwhile to other teams.