Tony Vargas
Legend
The simplest solution I ever saw to the TWFing conundrum was in the last Gamma World. The weapon list was very generic, just "light one-handed, light two-handed, heavy one-handed, etc...'Why shouldn't it be viable when a relatively simple solution exists. Multiple Bonus attacks without ability modifiers alone pretty much brings it into line, and also doesn't let it get so far ahead levels 1 through 5.
If you wanted to use a weapon in each hand, it had the same stats as a weapon that required both hands, because, really, you were doing the same thing - occupying both hands with wielding a weapon, so no shield.
From a completely different angle, I find the way they're boosting the fighter's DPR this time around interesting. In 2e (and later 1e), the fighter was just about broken in terms of DPR. Double-specialization applied to a bow or a matched pair of weapons, and the static bonuses from STR, spec, and any magic weapon(s) were multiplied by the large number of attacks (specialization started at 3/2, and TWFing could be interpreted as doubling that, like archery with it's RoF of 2 to start, so, 3 att/round at 1st level, which bumped to 4 at 7th and 5 at 13th).
What's the 5e fighter get? 4 attacks by 20th, and a 5th w/TWFing? In the same ballpark. All he needs is a juicy static damage bonus from 'specialization' or something, and he's back to 2e cuisinart-of-doom levels.
At the same time, casters also harken back to their most-broken days - 3.x, when they could cheese up untouchable save DCs (among other things). It's also not as bad, they can only get up to a 19 DC against non-proficient saves in the -1 to +5 range, but they can do that with every spell, not just their highest level ones.
Well, that's one way to 'balance' 'em.