One thing that's been evident for a long time is that multiple attacks that let you stack up bonus damage get out of hand very quickly. In 2e, TWFing plus weapon specialization (plus % strength/str-replacing-items plus fighter attack progressions) gave us the 'quizinart of doom,' outputting so much damage that no one else really mattered until mid levels. In 3e, TWFing, while nerfed, could still output a lot of damage if you stacked enough bonus damage onto each attack. In 4e, the Ranger is a top dog striker because of (deceptively) whimpy-looking Twin Strike, once you throw in a little static bonus damage, and 'crit fishing' with multiple attacks (single- or many-target) is also an overly-winning strategy.
Multiple attacks are an obvious, intuitive idea (and fun for the player that gets 'em), but they just may not be workable. Fixes I can think of would seem arbitrary and unintuitive, like:
Each attack that hits (the same target) adds [W] damage or a damage die, not bonus damage. And, only the first attack in a given turn determines whether a critical hit is scored.