For one particular question, most of the freeform answers will be one of a few possibilities, but there will be some outliers. Can I suggest these possibilities without limiting it?

Yes! Add a Special Instruction wtih the common answers. Workers will see that and it should help them.

Alternatively, if you are only interested in the few possibilities and not the outliers, you can define the field as a Multiple Choice, Select-One. The options can be the primary possibilities, with a final option for "Other."