Xeviat
Dungeon Mistress, she/her
Extra Attack, Unarmored Defence, that sort of thing. Although that's only surface level, and addressing symptoms rather than causes.
There are two solutions for Extra Attack.
1) let it stack, but add a 0 level to multiclassing (this also helps with proficiency gains and such). Even without a level 0, worst case is a 5/5/5/5 who has 5 attacks. I'd venture a guess that a Fighter/Barbarian/Paladin/Ranger isn't too effective compared to a Fighter 20.
2) Bring back BAB. Track combined levels for classes which give out Extra Attack and give it when you reach 5th level in them.
Personally, I wouldn't have designed Extra Attack into the game. The Rogue scales nice and smooth, while Extra Attack messes multiclassing up and makes starkly tiered scaling. Multiclass spellcasters get their higher level spellslots no matter what. Their cantrips scale no matter what. Weapon attacks should scale too.
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