The following players are eligible for the rule 5 draft if they are not on the 40 man this coming winter. Of note from that are likely the following...
Danny Lockhart, INF
Stephen Bruno, INF
Jeimer Candelario, INF
Corey Black, RHP
Pierce Johnson, RHP
Bijan Rademacher, OF
Ivan Pineyro, RHP
Daury Torrez, RHP
Dan Vogelbach, 1B
I'd also toss in Christian Villanueva who's on the 40 man but seems to have a tough route to the majors with the cubs. Bear in mind that Schwarber isn't presently on the 40 man roster and you conceivably could be seeing him at the end of the year or start of next. Baxter is an easy cut off the 40 man. Schlitter and Germen are likely too. After that it gets a bit more difficult. I'd imagine they could feasibly find spots for Pineyro, Johnson and Black if they choose to. Torrez is still in A+ so he seems a long shot.
Vogelbach seems like a guy that's gotta go. Not that he's a bad player but it's hard to believe he'll be much better than what he's hit in AA this year and rostering him just doesn't seem worth while with Rizzo. I think for an AL team he is probably good enough to be the second billing in a trade unless it's a Price level deal. For example, I'd argue he is more valuable than Olt was at the time of the Garza deal.
Bruno and Lockhart largely depend on what happens with Baez IMO. It is easy enough to ditch Herrera and go with one of those two. But between Alcantara, La Stella, Russell, Castro and Baez they are obviously super deep here. Bruno is a bit of an empty average guy however he has hit .309/.383/.428 over 903 minor league PAs. Not terrible for a bench guy. As for Lockhart I don't personally get why mlb.com has him on the top 30 cubs prospects but he is there. On Rademacher, he's another guy I'm not entirely sold on being in mlb.com's top 30 guys but whatever. If the cubs like him they could potentially move Lake and have him be a AAA guy for next year ready for call up. Jeimer Candelario is still in A+ having to repeat it. He might be safe from rule 5 because I'm not entirely sure a team would be ready to toss him on to their bench.
Overall, I think Villanueva and Vogelbach seem like the most obvious choices to go. Villanueva would be a decent cheap option for a team with 3B issues going into next year. He doesn't have amazing upside so you're not likely to get anything amazing for him alone but you could probably get a bullpen guy or a half decent OFer for him.