Bieber seems like a good and realistic option for adding a 2nd SP.
Otherwise? Unless you win the Sasaki lottery or make a HUGE investment into the rotation (e.g. a guy like Nate Eovaldi is your "other guy"), it's a bad idea.
Everyone is focused on the injury risk of the youths, and that's unfortunately a very real thing we need to worry about. However that needs to be balanced against the performance risk of this other guy. If Assad has a 5 ERA on Memorial Day next year, he probably gets a trip to Iowa, certainly at minimum he gets shifted to the bullpen. If you sign Michael Wacha to a two year deal and he has a 5 ERA on Memorial Day...nothing happens for at least another month. We can all scream "it's a sunk cost it shouldn't matter" until we're blue in the face, but you know how it works in practice with well compensated vets.
The Cubs next year have six optionable young SPs of consequence who are more or less major league ready. Each has warts, and there's no law against multiple concurrent injuries, but I think it's much more likely that we get positive production out of the 5/6/7/8/+ rotation spots by rotating the youths than we gey by locking another spot down with a vet. Add in the opportunity cost of that added salary and it's clear cut IMO.