Rules Patch: Army Composition v8.1

PROBLEM: while large units of weak troops are and should be a staple of the game, large units of elite troops, especially when carrying "certain powerful magic banners" (you know what I'm talking about!), become unforgiving "death stars". These concentrate most points of the army into very offensive but also very defensive points, which make for extremely difficult games with few options left for the opponent.
Additionally, many of an army's most powerful units come from the Rare section, and very often whole army builds are designed around them (including 2 copies of the same rare unit to maximize their benefit). Not only does this make for very difficult army builds (as some Rare units can be very very powerful centerpieces), but also breaks the verisimilitude of seeing 2 Rare units on the same battlefield more often than you would see 2 of the same Special unit.
SOLUTION: any single unit (before any characters are added during the battle) cannot exceed 20% of the total points of the army (this would be 500 points in a usual 2500 points army). Only 1 of the same Rare unit and 2 of the same Special unit may be taken in a normal-sized army (up to 2500 points). These values are doubled for Grand Armies (2500+ points).

This simple rule change can single-handedly remove many of the more problematic army builds that have plagued 8th edition, such a 2x Terrogheists, 2x Hellpit abominations, etc. 
It should also curb some of the most powerful elite units from turning into veritable death stars, by preventing them from increasing their numbers exponentially and on top giving them expensive magic banners (which normally creates a unit that either is as destructive as it is impervious to damage, or simply so destructive that it will gain so much Combat Resolution advantage that it is unlikely to lose combat).