I would say subjects are obligatory and take ni, ona, ijo when the subject is "weak" I think most people have the weather doing what weather does, walo lete li anpa. I think there are discussions of attempts to make passive constructions (without a "strong" subject) elsewhere.
Maybe there's another transformation rule?
sina o pali e ni! => o pali e ni.
If a verb is the answer, I'd be inclined to add li.
Q: jan suli li seme e jan ante? He did what to the other guy?
A: li moli! I suspect most people would say, ona li moli, I can't remember anyone writing a li+verb fragment, maybe it is because I don't hang out in the chat room.
If a prep phrase is the answer, I'd be inclined to include the preposition.
Q: jan li lon ma seme?
A: lon ma Mewika.
But ma Mewika sounds find, and just Mewika sound like fragment the same as the others.
And for all I know for these possible rules of fragments, maybe I'm just rediscovering how English works, similar to how I noticed people order their tp adjectives in a certain way, which happens to mirror English.