Malaconda is in a weird position. As things stand now, it will probably be the single worst CAP post-update if it doesn't receive any significant changes. Malaconda is basically usable now: it has pretty good typing (seriously! It should be able to switch into Colossoil and Cyclohm and Krilowatt and Latios and Zygarde...), and when you max out Defense (as in, max HP max Def) it actually survives even unexpected stuff like non-STAB U-turn from physical attackers. The big issue to me is that it's outclassed pretty badly by Colossoil: it can't break anything that Colossoil can't, has the same counters, does way less damage, and is much slower. It's best niches are Spin and Glare, and against a team with, say, Pyroak or Tomohawk, it will probably do nothing, so one can argue that Toxic is better than Glare for it, and at that point why are you using Malaconda?
First off, I want to say that I think reachzero's first paragraph does a good job summing up Malaconda's place in the current meta. It is not terrible. It is not unusable. It is just niche, and has issues with certain Pokemon that make it have major problems with certain matchups, even if playing to its niche. It can certainly fit on a team, and its not going to be awful. But it is just lacking compared to a lot of alternatives.
That being said, my biggest problem when thinking about Malaconda is that I really have no idea how to best improve it. It's got some really solid parts, but its whole is just not up to snuff in the metagame. My initial thoughts definitely mirror Deck Knights, in that I think Harvest is still a very solid ability and doesn't really need an upgrade, and that the slight downgrade to it with the loss of perma-sun simply means we can afford to be a bit more free with movepool options. That said, I'm definitely willing to consider alternative approaches with ability if it seems like a good way to help Malaconda. But I'm simply not sure that will be the case.
Some suggested abilities I am highly against, chief among them being Regenerator. I do think that Kyubics basically summed up my feelings on this ability already though:
One ability that I am not a huge fan of is Regenerator. I don't think that recovery is something that Malaconda ever has, or ever will, struggle with and it feels like people are just throwing a generically good ability at the problem and hoping that it will be solved.
Regenrator is a great ability.... but it really is not helping Malaconda. Malaconda has a fantastic ability for recovering health, alongside options for reliable recovery. Keeping its health up is not really an issue for it. I guess Regenerator would let it do so with a different item, but I don't really see that as addressing any issue Malaconda has. All it is really doing is encouraging specific kinds of sets (ones with U-Turn), when I don't really think those sets will really be all that much better than they are currently.
If we are going to give Malaconda a new ability, I think it either has to provide new utility Malaconda doesn't currently have, support its team, or boost its power. Intimidate is one that fits into the former category, as it would allow it to switch into a number of physical threats that it couldn't before letting it function in more of mixed defensive way. I am not entirely convinced this would help it so much, as its biggest physical weakness if U-Turn whose users are typically happy to switch in on Mala, and Mala is not going to be switching into. But, that being said I do think it addresses something that could potentially help boost Malaconda's standing in the Metagame. I just don't know by how much.
Drought, which was suggested by reachzero, is more of a supportive ability. Yes, it helps with recovery via Synthesis, but as I already said above, Mala doesn't struggle with recovery. But, unlike Regenerator, this would actively improve its role in the metagame by allowing it to support other Pokemon. It may not be the next Mega Charizard-Y, but it could better support full sun teams, as it can hold Heat Rock. It also has reliable recovery and good utility moves, making it a huge step up from either Ninetales or Torkoal. At the same time, as reach said, doubling up on grass types with Venu is definitely not ideal, making this unlikely to be an overbearing ability for it. I'm not sure this is how I would like to go about trying to solve Malaconda's issues, but I do think it would certainly be an improvement for it.
As for the last idea, a power boost, the only ability I have seen brought up so far in that respect is Strong Jaw. While I very fitting move, it really doesn't currently have the movepool for it, having only Crunch. That being said, if we decided to increase its offensive options, this could actually be a very solid ability. Strong Jaw's 50% boost to Crunch, alongside potential Elemental Fangs would go a long way narrow the power gap between Malaconda and others such as Colossoil. However, it would not really help a number of its other great moves, such as Power Whip, Knock Off, or U-Turn. As a potential alternative here, I think Adaptability might be worth a thought. It cannot help with coverage, but it is a huge boost to the power of its main damaging options, and makes switching into it much harder, without being too overwhelming. At the same time, it does not really require any changes to its offensive movepool to make the most of the boosts.
In the end, I'm really not sure whether a new ability is even the right way to go for Malaconda, but I do think there are some potentially good options out there. I'm just not sure what is the best approach.