Alright, lenghty post incoming.
This thing is quite the enigma for me, since I just can't think of any perfect way of increasing its viablity or just make better in general. It has all it needs to be a good utility mon, it has acces to realiable recovery, its defenses are actually good minus the physical one, but the HP somewhat makes up for it, yet it falls flat on its face with what it's trying to do. These are more or less my suspicions, I wasn't here when Perma Weather was a thing so I can't say how good or how bad Malac was during that time, but there are probably a few reasons why Malaconda just doesn't work anymore:
1. It was made for a mediocre weather - when it comes to weather, the most common ones (and for good reasons) are Rain and Sand. This is mostly due to the sheer diversity they present, how there are Steel and Ground mons that become stronger or faster in the sand or how Rock mons can take hits better despite there own type not being so great defensively, even in the Rain you have mons that take full advantage of the effect, loosening there Fire weakness, having a perfect accuracy Thunder and Hurricane and just great team synergy all around. Sun is not really in the same category or at least, not anymore. Outside of Fire types that set up the sun or just Fire types in general, the only mons that have abilities that can take advantage of said weather are Grass mon.......and we all know how amazing the Grass typing is, right ? Sure you have Mega Houndoom and Regular Zard with Solar Power, but every other mon that has a weather boosted ability is a Grass mon, a type that's WEAK to the one that's powered up by the weather. Yes you have Chlorophyll so you can boost you low Speed, but then you ask yourself, what do I hit my opponent with ? Unless you're a insanely hard hitter like Bulu, with your own way of boosting your STAB, then that Energy Ball or Solar Beam won't even leave much of an impact, especially if your opponent resists Grass moves. And Grass also suffers from terrible coverage, the most ammount of coverage are given to physical attackers while special ones are left with stuff like HP. And what type of move is Solar Beam ? Oh yeah a special move that requires the sun to work, a move that only mons with barely any coverage can properly use. I don't want to talk about this any further cause otherwise I would end up writing a novel and there is still more to talk so I'm just gonna put my main points out: Sun is just a mediocre weather that only became apparent after its nerf in 6th gen and its glearing flaws started to show up (or they were already there and evryone already knew about them, but maybe became better in Gen 5's meta, idk), the type that's supposed to work with the weather simply doesn't because of just how weak of a type it is and really, the only type that takes real advantage of the Sun is Fire, similarly to Ice and Hail. And Malac, while it might have been a good team mate for sun teams, that was then, now with infinite weathe gone, it can't do it's intended job anymore or just as good as it used to.
2. Its typing - Grass is a type that is really not that great offensively, but can shine as a defensive one due to its unique set of resistances in Ground, Electric and Water. Problem is you have to know wich typing it can work with, Grass/Steel is a good example of that. Grass/Dark is not. There's a reason why the only two Grass/Dark mons we've ever gotten, Shiftry and Cacturne, are offensively oriented. Yes Cacturne is slow, but its overall stats clearly show that it's meant to be a attacker. Same with Shiftry. In fact, most Dark type are inclined towards offense since that's pretty much what the typing itself is made for, to be a offensive one. The only reason why defensive Dark mons like Alolan Muk work is because of there typing being pared with a already good defensive one (like Steel or Poison, ok Poison is rather mediocre, but you still get my idea). Grass is a type that needs a defensive type to work, unless you already have good stats like Tangrowth then you can work even as a mono. Dark is not one of those. And the laundry list of weaknesses that come with this combo aren't doing a favor either. Sure T-tar has its own multitude of weaknesses, but it's high stats and ability work together to compensate alongside a diverse enough movepool. Malac sadly doesn't get all that privilege. Just like Heal said, Malacoda is a pokemon that's supposed to stay in long enough, but its multitude of common weaknesses prevent that from happening.
3. It's just outclassed - I don't think this comes off as a surprise, as a defensive Dark type, Collosoil does its job far better, as a defensive Grass type, Tangrowth invalidates it, as a offensive mon, [insert any Dark or any Grass mon with a offensive presence here].
I know that by now I might've skinned the snake to the bone, but these are just a few observations I've made over this thing. It can't work as a purely offensive mon because of its pitiful attacking stats and speed and it struggles as a defensive mon with so much competition. And Sun is just bad.
So for abilty suggestions, I'm most likely just gonna try and focus on the role that many can see Malaconda realistically accomplish: a defensive/utility mon. And also some possible commentary:
Regenerator - it's not to farfetched when you think of a snake that can heal itself once it's out of battle and it helps it not being to reliant on Harvest (which I think it's a very iffy ability). But Malac's entire purpose was to be a mon that can work in the sun, I think keeping accent on Harvest would still be required so I'm in a gray line when it comes to Regenerator.
Natural Cure - halps deal with status, especially is it needs to switch a lot of times
Filter - not being so afraid of all the common weaknesses it has
Sap Sipper - a added immunity that raises one of its stats can prove to be usefull
Rough Skin/Iron Barbs - can help with its low defense and not make U-Turn so spammable
Bulletproof - can say that Malac has thick, treebark like skin (its supposed to reference the Fruit of Knowledge tree if I'm not mistaken) that offeres it extra ammounts of protection.
Intimidate - sounds a bit broken, but with the defensive nature that Malac has, I can see it definetly work, plus that intimidating look that it has.
Drought - this one I'm not a fan of, the mons that set the Sun are already good enough (Zard Y with its raw power, Ninetales with its speed + Nasty Plot, Torkoal with the utility it posesses) and having two Grass mons on the same team is not really a good thing, especially one that invites so many things in. Plus, the only other pokemon that got Drought was Groudon and even then it becomes a Fire type when it Primal reverts (seriously I always tought Groudon was a Fire type even before ORAS). Plus, what exactly even says "intense sunlight" about Malaconda ? Pelliper getting Drizzle made sense since it's a pelican and those things tend to live in ports and tropical lands so a heavy storm is always to be expected, as for Gigalith, well it's a living rock formation and those can be found all around the world, even in deserts. Malaconda is just a grass snake that references a biblical story. Not much else going on for it.