Oops, you gave three hints. All of them are wrong for this sentence. Be careful not to confuse "fica" with "tem", "recebe", or "apanha" It will confuse the student and lead to frustrations with which you would not like to deal.

So why not "Passa ser mais interessante", but only "Passa a ser..."?

OH NO! This sentence completely threw me off course =)

I have not as yet come across Vira, and had completely Isto forgotten about isto, (I still don't really know when to use isto)

how about "chega a ser mais interessante"?