implicit none is a good start, it is especially annoying for interfaces.
B.3 Obsolescent features contains already a list of features that are identified as outdated, the most “popular” ones are probably common blocks and fixed form.
A clear procedure with a clear deadline (e.g. next standard revision) for removal would IMHO be better than waiting until almost everyone found the time to modernize their codes.
Consistency of type/rank/kind as discussed here is also something that should be fixed. Too bad that typeof made it into the standard, but even worse if this glitch will stay forever. Yesterday I’ve started discussion on Interface definitions for assumed size arrays, also something that could be improved. Maybe there is no better solution, but knowing that I won’t be fixed even if there would a solution that is superior in all aspects but backward incompatible is weird.
I also agree with the Zen of Python: “Namespaces are one honking great idea – let’s do more of those!” and would like that use math makes functions available as, e.g, math%eigenvalues instead of cluttering my namespace. Compatibility with existing code would then require something like use math as *. Of course, one could make the better option opt-in (as with implicit none) but I think a good language has reasonable defaults and the option to do stupid things instead of stupid defaults and the option to do things right.
Edit: I forgot the newest flaw enumeration and enum: Flaw with typed enumerators in F202X. This should be straightened before the new feature finds widespread use.