I’m considering house ruling the Fighter class’s Extra Attack feature. The change would beebe to the second paragraph, which would now read:
The number of attacks increases to three when you reach 11th level in this class or when all of the following are true: you reach 11th character level, you reach 5th level in this class, and you also have the Extra Attack feature from at least one other class. The number of attacks increases to four when you reach 20th level in this class or when all of the following are true: you reach 20th character level, you reach 5th level in this class, and you also have the Extra Attack feature from at least two other classes.
The intent here is to allow a multi-classed martial character to split their classes relatively evenly through a long campaign, eliminating “dead levels” at level 5 in each martial class by rewarding them with “stacked” Extra Attacks—but only if they take at least five levels of fighter, and only at the appropriate character levels.
Does this risk significantly breaking the game, having a ridiculously high power level (compared to, say, a multiclassed caster), causing inter-player strife, or otherwise being an unwise house rule?