"That is a very good idea!"
Translation:Tio estas tre bona ideo!
I was tempted to put 'tiu' here because I assume that the speaker is referring to a specific and known idea. Do you think that would have been accepted?
Tiu is wrong because it always goes with a noun. Tio is a pronoun and can stand in for a noun.
I don't think it always goes with a noun. Sometimes it can implicitly mean something kind of like "that one", without the need for something in the place of "one" like in English. It might be a case of ellipsis; not sure.
That said, I would not use "tiu" here because it sounds like you're picking one out of a number of ideas, which is probably not the most likely interpretation, even if you take the plans to be hypothetical alternate plans that could have been suggested. I would not say it's strictly wrong, however, and do think it should be accepted. All the same, I suggest using "tio".
I've reported that sentence as "my translation should be accepted".
If you think your translation should be accepted or you find any other mistakes in the sentences, it's better to use the "Report a problem" button than write a comment in discussion. :) The developers can see your suggestion much faster and easier through problem reporting.
I appreciate the advice. I will try to be more confident about the acceptability of my answers. :)