I'd be shocked if the team adds another guaranteed roster spot SP. Snell/Montgomery seem unlikely for financial reasons, any trade target will only be nominally available before July, and Lorenzen/Clevinger probably aren't enough of an upgrade over the kids to be worth it.
I could see someone in the Eric Lauer, Brad Keller, Matt Boyd kind of mold on a MiLB deal. If you're one of those guys you have to balance the strength of the Cubs' pitching dev vs. the strength of your competition for a roster spot. I have liked Lauer for a while (he was basically Aldi brand Imanaga pre-injury), and I remember a fairly compelling a Twitter thread from Greg Z about Keller.