I know newsworthy is used to describe an event that deserves reporting (or to be reported?), therefore I wonder if I can describe a question worth asking in this forum questionworthy. That's completely based on my own logic and I have never heard of it before.
So, is questionworthy a usable word? Should I make up words in my writing this way?
The general principle is obviously that learners should be extremely cautious about "inventing" new terms, because they're so much more likely to get things wrong than native speakers.
Here's the full (subscription-only) Oxford English Dictionary definition for the relevant suffix...
-worthy (combining form)
Forming adjectives with the sense ‘deserving of what is specified by the first element’, as blameworthy adj., noteworthy adj., praiseworthy adj., etc.
Although I can easily guess1 what OP intends his neologism to mean, it should be fairly obvious that it doesn't really fit the pattern of the three examples above, which mean should be blamed / noted / praised.
The existing English word for the corresponding should be questioned sense is questionable, but that's obviously not the meaning OP intends.
It certainly doesn't help that blameable , notable, praisable are all "more or less" acceptable synonyms for the OED's examples, but things fall apart when we try to apply the pattern to question (questionworthy can't mean questionable).
In any case, apart from "That's a questionworthy question!", I can't really see where OP's neologism might be useful. And as all politicians know, the idiomatic standard there is "That's a good question!"
1 Thanks to @Lambie's comment, I now realise there are at least two possible meanings for OP's neologism questionworthy (describing an actual question which is worth asking, or someone's presentation which merits the audience asking questions afterwards).
Which just goes to show what can go wrong when you make up words, even if you think the meaning is obvious!
English usage gives one the freedom to create new words by analogy, especially if no apt word already exists. Often when such coinages are made the coiner will put the word inside quotation marks on its first use in their text. Be careful, though, as such coinages can be cringeworthy.
You can't make up a word without defining it. So I would only make up a word if I were going to use it repeatedly throughout a document. The first time you use it, you should define it so that everyone understands exactly what you mean:
Let's call a question that is worthy of being asked (e.g., meets the forum criteria, is not trivial) "questionworthy."
Then you can use that word as much as you want in the rest of the document. If you're only using it once, since you have to define it the first time, it doesn't really make sense to create it.
Valid might be a good substitute (a valid question).
Of course, all words must have been made up by someone at some point in time. But as an English learner you normally avoid this.
In the particular case "questionworthy" already has some use. It is used to describe something that worth asking a question about.
One weird character was "Pops". Let's ignore the questionworthy relationship he had with his student,...
(The speaker suggests it would be worth asking the question "is this relationship ethical?") If a native speaker used it like that I'd probably just think it was a bit odd, but it makes sense and not think more of it. If an English learner did the same, it would seem like a mistake.
Creating new words is "incorrect" from the point of view of tests and there is too much risk of making your English sound unnatural. So I'd have to recommend not creating new words. Instead, you should combine words. You can say. "Something that is worth questioning". You are then using standard English, and are clearer. This is better English.
