I suggest the ability to change the head type in a engine family.
Because this thing is too common.
For example, the C20LET and C20XE (the first a SOHC 8 valve, and the other the classic DOHC 16 valve), or the toyota with the 4A family (the 4A-GE it was DOHC, available with 16 and 20 valve, and the 4A, 4A-C, 4A-L, 4A-LC… it was a SOHC 8 valve).
To produce a engine family is very useful.