You can always go for big minors that can eventually become very strong:
- Poland
- Romania
- Spain (after you get past the civil war)
- Mexico
And if the RNG gods favor you and Czechoslovakia submits, possibly the best minor to become a major: Austria-Hungary
The only issue with that is that Czechoslovakia won't always accept.
When you take "Protect Czechoslovakia" either:
- They want to become your puppet (worst outcome, 50% chance)
- They refuse and you have a wargoal on them (30% chance)
- They accept and integrate into Austria-Hungary (20% chance)