How to improve: 331-332-Cacnea-Cacturne:

1.Stats: Should Cacturne get a stat boost, Evolution, Mega Evolution or nothing?
Cacturne's primary reason of existence is to be a early mid--game Grass-type. Its secondary abilities include being the original owner of Needle Arm and being the only Pokémon in both Human-Like and Grass Egg Groups.

This, is not big. Needle Arm hasn't been relevent since it was invented, and there aren't much Grass-types in Human-Like and humanoid Grass-types in Grass Egg Group. This leaves Cacturne with only small niches.

In terms of a stat boost, Cacturne can get a few boosts, ranging from 10 to 40. However, due to its stat distribution, this will not help much.

Mega Evolution would be better, especially with a small boost. However, Cacturne's stat distribution makes it incapable of being used in a traditional team, even with a reliable priority move. So, either a massive redistribution is necessary, or we need some support like Trick Room or screens.

Nothing would be terrible, and probably wouldn't make much sense. Cacturne's 475 BST is not that large, even for a two stage Pokémon.

2.Should Cacturne get more abilities?
Yes. It has only two abilities so anything can work.

Sandstorm based abilities such as Sand Rush and Sand Force can work.

Utility abilities like Prankster and Technician can work.

Grass based abilities like Chlorophyll can work.

Poison based abilities like Poison Point, Poison Heal and Poison Touch can work.

3. Should Cacturne get more moves?
Yes, particularly Ground-based moves, like Earth Power, Sand Tomb, Stomping Tantrum, Poison moves like Gunk Shot, and powerful Grass moves like Petal Dance and Petal Blizzard.

Punching moves can also work, but Cacturne already gets a few. Bringing them back to a more easily accessible form would be good.

X-Scissor would also be good. Toxic Spikes and some Ghost moves like Hex, Shadow Sneak, Phantom Force and Shadow Ball would be good as well.

4.What additional typing Cacturne can have?
Ground, Poison and Fighting are the primary alignments for Cacturne. Since Cacturne already has a rare dual type, such an idea can only work for Alternate Forms.