The Orioles center fielder hit .291/.360/.518 this season with 30 HRs and 37 doubles, and stole 30 bases for good measure. In a slightly funny stat this resulted in just 59 RBIs as he hit at the top of the anemic Orioles lineup. The Orioles are building for years down the road and they might decide the most value that the emergence of their 27-year-old CF could provide is for bringing in even more young talent to their pipeline. It's hard to say for sure if they'll entertain offers but assuming teams will ask, the Reds should be one of them.
What I like about possibly acquiring Mullins is that while it would certainly take a package of young talent to land him, it would likely come cheaper than some of Mullins peers at the top of MLB hitting categories on account of Mullins lack of a track record. It may have been only one great season so far for him, but it was a one where he hit well in the first half (.921 OPS) and in the second half (.822 ops), and even held his own against LHP as a LH batter (.788 OPS). Some metrics don't love his defense but nobody really questions his ability to play a strong center field. This is a guy who was the top outfielder in the AL in fWAR, and he's up there in most major offensive categories.
Now if the O's were prepared to listen on him, I'd probably put most guys on the table with few exceptions. I'd probably want to keep Hunter Greene, but beyond that I'd say make recent first rounders (Lodolo, McLain, Hendrick, etc) available and see how things could match up. It would be risky, I'm always leery about these guys who come out of nowhere and suddenly hit like crazy, but the reward of a young, still cheap player to solve the CF issue, pair with India at the top of the Reds lineup, and boost offensive production significantly would be extremely high too.