With Gundo allegedly off (if we're negotiating with Sporting for Fernandes, that's a clear indicator it's happening) and Silva regressing at an alarming rate, I think going into next season with KDB, Bernardo, Silva and Foden is a bit of a risk, as good as it looks on paper.
KDB has been plagued with injuries and as much as we want to avoid it, I'd be surprised if he doesn't have at least a spell out next.
As I mentioned above, Silva just isn't the same player right now. A summer off may do him the world of good, but he's at the age now where performances can dip off and never recover.
Bernardo has been favoured on the wings recently and I can continue to see Pep view him as one of the key options there, especially if Sane leaves us because that will mean Sterling will see more time out on the left (though I do anticipate we'd replace Sane with someone like Felix).
Signing another CM would impact Foden's opportunities, particularly given Fernandes is only turning 25 in September. But I think Silva only has one more season in him and that's going to be much more limited in any event in terms of the number of appearances he'll make. Foden will be 20 when Silva will probably go - he'd still get plenty of chances as 1 of 4 CM options, particularly if Bernardo is seeing a good chunk of his game time on the wing.
Ultimately I think we have to be looking at signing someone if Gundo were to go, it just represents too much of a risk not to given the variables. KDB gets another 3 month injury, Silva's legs continue to decline and we're suddenly left with Bernardo and Foden playing every game.