Is there currently any way to get an expanded critical range (allowing a critical hit on a 19 on the die, or even lower) or a higher critical multiplier (at least x3) on weapon attacks?
There is one way to get an expanded critical range - the Champion Fighter's Improved Critical feature.
Beginning when you choose this archetype at 3rd level, your weapon attacks score a critical hit on a roll of 19 or 20.
Which later becomes Superior Critical:
Starting at 15th level, your weapon attacks score a critical hit on a roll of 18–20.
The Hexblade's Curse grants them Improved Critical against a single target (for...some reason).
There is no way to get a higher critical multiplier. However, there are 2 methods of adding the weapon's damage die an extra time on a critical. This is analogous to a x3 multiplier, or x4 if combined, but only multiplies weapon damage, not any other source of damage. The half-orc's Savage Attacks feature:
Savage Attacks. When you score a critical hit with a melee weapon attack, you can roll one of the weapon’s damage dice one additional time and add it to the extra damage of the critical hit.
And the Barbarian's Brutal Critical feature:
Beginning at 9th level, you can roll one additional weapon damage die when determining the extra damage for a critical hit with a melee attack. This increases to two additional dice at 13th level and three additional dice at 17th level.
Meaning that a half-orc Barbarian 17/Champion Fighter 3 gets a crit on a 19-20, and deals the weapon's damage die 6 times over on crits. Note that this doesn't work quite as well with weapons that do 2d6 damage.
In theory, your GM might allow you to stack the expanded critical abilities if you multi-classed 15 levels into Champion fighter and 5 levels into Hexblade warlock. A GM might allow the improved criticals to stack (17-20), and if you get the right enchantment on your weapons it could go up. Granted, that is all the GM's choice; talk to them about it.