If GW was to redo the Warhammer fantasy rules then here are some suggestions as to what would be good to include.
Fliers should have a 10" failed charge move.
All movement would be done by paths which would simplify redirection and enemy in the way. These paths would be the width of the moving unit and in cases of charging the path would move roughly from center of unit through center of unit.
When skirmishers are charged the charging unit determines the path any way they want that its path moves though a skirmisher.
When fliers charged only those facing the target would get into combat and those skirmishers not in combat would not count towards unit size.
Frenzy would be written better. I would give examples to show how frenzy could fail.
Instead of having fear auto break smaller units fear would give a +1 to combat resolution if the fear causer is smaller, +2 to CR if the fear causer is larger, and +3 for terror causers.
It would require US 5 and at least 3 models in a unit to take away ranks.
All army books would be done simultaneous so they would be roughly balanced.
The rule book would have lots examples for situations which are not trivial.