Are you suggesting it's going to look a lot different from the previous 45 years? Because the previous 45 years suggest pretty strongly that we do in fact know how to design a block cipher.
Public knowledge of differential cryptanalysis is younger than 45 years, and was known to NSA 20+ years before its public "discovery" (and known to IBM 15 years before).
Multiple finalists in the AES competition (which selected Rijndael) had significant weaknesses found in the decade since, despite receiving relatively little attention (MARS, Serpent).
Assuming that A and B have received an equal amount of cryptanalytic attention and the "best publicly known" attacks represent equal amounts of effort/progress against the algorithm.
In practice, we need to overspecify algorithms initially, not knowing what the next 10-50 years of attacks will look like. Maybe 15 years in you go "woops, could have used fewer rounds-- we're holding up much better than I thought," but at that point the parameters are standard and...