Should I submit my revised workshop paper to a journal if I don't think it will meet the "30% new material" requirement? Let me explain my case first. I submitted an article to a Computer Science Conference, and it got one rejection, and two qualifications as a borderline paper. The reviewer that rejected the paper only said that it was not in the scope of the conference and nothing more. The other two reviewers made thoughtful comments and the final verdict was that I should submit it for a workshop on that conference.

I made the necessary changes, submitted to the workshop and it got accepted. Here it was one accept, one borderline and one reject. So I changed some parts that the reviewers suggested and submit it for the final printing.

The thing is that there will be an special edition of a journal that is planning to consider the papers submitted to this conference. So the authors should re-submit their papers for a new review and they state that the papers should present at least 30% of new material.

Here is the point, the deadline is approaching fast and I am making the add-ons based on what the reviewers point me before (when I first submit it for the conference and then what was the suggestions for the workshop), but I am running out of ideas; by the way, I am the sole author of this paper. What should I do in this case? Should I just submit it with the changes? I just don't know if that would be enough. Or am I just wasting my time and should I left it because it has already been published in the workshop?
From an ethical point of view, submitting your paper is perfectly fine if you make clear what the changes to the workshop version are that make the paper contain something new.

As far as the question whether you should do it is concerned: If you cannot even convince yourself that you have enough new stuff in the paper to justify (another) journal publication, then it will be insanely hard for you to convince the reviewers that there is enough new stuff. So the submission is likely to waste your time (and possible the reviewer's time -- if the paper gets past the editor) as it is too likely to be rejected. If you are very lucky, some reviewer will suggest a possible point of extension that will make the paper strong enough for another submission.