What alternative to wild swing combat in a 2d platformer type game like starbound is even possible?
Apart from the other suggestions, we could hop into the Wayback Machine, and take a look at how the 1989 Prince of Persia handled a deadly and reflex-intensive fencing-style combat system, despite their technical limitations, serious frames-per-second limits, and lack of game design precedent to pull from.
They created a simple, fast, and surprisingly deep 2-button combat system, with an Attack and a Parry button, and basic forward and back-stepping mechanics, designed to simulate the movements and techniques of fencing in a simplified form. The attack animation has a tiny warm-up; if you hit an enemy while they were preparing to attack, their strike would be interrupted and they'd recoil a bit, preventing you from being hit. The warm-up time on Attack also gives the defender a cue to quickly Parry; if the enemy is Parrying while you attack, your attack is deflected, and you have a short recoil time during which the defender can quickly follow up with a free hit. Parrying has no warm-up time, but instead leaves you vulnerable and unable to act for a tiny bit afterward, meaning you can't spam Parry without the enemy timing an attack to come in during the cooldown after your Parry ends.
We see a few straightforward fights at the 13:53 mark. Though it's tough to see how challenging it is when you're watching experienced players instead of playing it yourself, you can see the mechanics in action. When the swords tip up, that's a parry. Successful attacks make a Thwap sound, and Clanks mean an attack was successfully parried.
At 15:56, we see the Fat Guard; they parry almost perfectly, so the only way to hit them is to parry their strikes, and quickly counter-attack, which takes serious skill and reflexes.
Full Playthrough:
https://www.youtube.com/watch?v=Xv20j8ChtRYIn short, you could create a system that emphasized timing, positioning, and reflexes, by creating a front-loaded delay on attacks and a back-loaded delay on defense (preventing you from holding it down), and a recoil time when successfully hit or blocked against. Looking over the dev blog, they're planning to implement something like that already, but we'll see how they adapt the idea for non-linear attack angles, ranged combat, and relatively more quick, jumpy characters.