I was to quote the old saying: "never change a winning team". But as Opeth has already proved that phrase wrong (by changing half of their "winning team" after Morningrise), I won't say that.
Anyway, I must say that I don't see any reasons for Opeth to drastically change (or "improve") their sound towards any direction, as they have clearly already found themselves musically. There are examples of bands stagnating to one musical concept and repeating it over and over again, until the band-members themselves get bored of it (often a lot after all the fans have already abandoned the band - exceptions like AC/DC prove the rule), but I can't see Opeth falling prey to such temptations - their sound, although recognisably similar on each album, is still each time so refreshingly "new" and their music so varied and surprising, that there's no fear of Opeth repeating itself to death.
I agree that there are bands that have used keyboards effectively to create _MUSIC_, even bands who have succesfully changed their sound between albums by adding keyboards to where there previously were none (Dark Tranquillity, for example). But, I say, Opeth don't _NEED_ any keyboards - they have enough everything in their music already.
-Villain (np: Dark Tranquillity - Projector)