1. Tee Higgins
Until his contract situation is resolved, up to and including actually signing his franchise tender, Higgins is theoretically available. The Bengals don't seem at all inclined to engage in talks on a long-term deal, with reports no discussions have been had in basically a year.
With the extra cap space they've cleared (once official) tied to voiding Cameron Sutton's $10.5 million salary for this year, the Lions have plenty of room to take on Higgins' $21.8 million franchise tag for this year. A move to get him would arguably give them the most dangerous offense in the NFL, with a wide receiver trio of St. Brown, Higgins and Jameson Williams. Keeping Higgins beyond 2024 wouldn't necessarily be out of the question either.
The Bengals may be losing leverage by the day. Higgins is literally not able to be at voluntary OTAs or any offseason work, since he's not technically under contract right now. He can just choose to stay away. The two sides have until July 15 to reach a long-term deal, but as noted that's got a very slim chance of happening.
Cincinnati could rescind the franchise tag, which would make Higgins a free agent with naturally fewer options than if he had just been allowed to hit the open market in March. Which given the way the Bengals have handled players they franchise tag lately, is probably the point. Essentially, "you'll sign the tender and play for us, or you won't play for anyone this year."
Until something changes for him, and maybe even after (if) he signs the franchise tender, Higgins stands as the most interesting external option for the Lions to replace Reynolds. Not necessarily the most likely, if that route is even pursued, just the most interesting and without question the best.