I would like to include a sentence in a response to a critical reader thanking him for his "fertile" comments, in the sense that they lead to improvements of the text he read. "Fertile" in a figurative sense would seem to me appropriate, however the dictionary entries for this word don't include a figurative meaning going that far (an investigation may be fertile, but not something that a comment or an idea it seems — and a google search seems to agree with that).
What word would you use? 'Fruitful' does not sound appropriate either, because it is not the comment that actually "bears the fruit", but only the work initiated by these comments. I would like to avoid less focused qualificatives as 'intersting comments'.