I'll start: what it would it take for the political position of Plaid Cymru to seriously improve, as occurred for the SNP?
1.5 million people wake up one day to discover that they have suddenly become fluent in Welsh.
While this thread got labeled 'the thread for ignorant Americans', you'll of course find that lots of British people don't get this and sometimes in fact actively refuse to.
I suppose Plaid Cymru could also completely change how they campaign and what issues they focus on as an alternative, especially together with an extraordinarily unpopular Labour Party government. Even then though, I think it would take at least a generation before the Plaid transformation started to show dividends.