Personally, some abilities should not exist in Pokémon. Therefore, let's give them new ones.
Illuminate: It does absolutely nothing whatsoever. You hear me? Nothing.
Klutz: The Pokémon can't hold any items whatsoever.
Normalize: All moves become Normal-types. That's all it does.
Swarm: Just IMO. Most have better abilities
Ledian (Swarm (It's weak already, as if the ability isn't enough))
Volbeat (Illuminate, Swarm (That's right! Both! Volbeat really needs to be redone.))
Buneary (Klutz (Hinders the fact that Buneary needs 255 happiness to evolve))
Lopunny (Klutz (Even though it already evolved))
What abilities, other than their hidden abilities, will improve those Pokémon?
I don't see swarm as useless. Some of the Pokemon listed aren't too great in general due to their horrible stats. Swarm actually helps them using their already poor stats to boost the ability faster.
Illuminate raises the chance to find Pokemon in the wild. It has its bonuses even if its not for battle.
Kluts has benefits for item switchers. You can easily mess someone up by sending out a Kluts Pokemon with Choice item and force switching the Choice Item to the opponent, or even using Fling. Give the Kluts holder an item that is usually bad for it like Toxic Orb or Iron Ball and Fling it at an opponent.
Normalize loses SE but gains STAB. With only 2 resistances which even out the damage, Normalize makes it easier to bypass a lot of other issues. It has its own perks if you are willing to overlook the set back of no SE hits.
Illuminate should have an effect that activates during battles, but I'm not sure what it should be. Stench and Pickup were given in-battle effects, so I don't see a reason not to give Illuminate the same treatment.
As XanderO pointed out, Swarm isn't useless entirely; it works in the same way as Overgrow, Blaze and Torrent, but for Bug moves. It's viable on Scolipede because it can spam 270 base power Megahorns (taking STAB and the Swarm boost into account). The same applies to Escavalier.
You could give illuminate a 50% chance to lower acc each turn. Makes sense.
But you can still hold items with klutz you just don't gain their effects. But an effective user of klutz is loppuny. It can effectively use fling and switcharoo(I think) with hazardous or dehabilitating items such as flame orb, iron ball, or choice scarf without worry about affecting itself.
For normalize you can make it so that all move targeted at it is turned to a normal type move instead of the other way around. No weaknesses and removes STAB boost from opponents.
Ya Swarm is the Bug version to the Starter's abilities. I think there is another similar to them but I don't remember it.
Run Away has its in game uses. You don't want to battle, having little health or no quick way to the Pokemon Center and need to get to the next area. Run Away has the same 100% Flee chance as Smoke Ball so you always run away. It saved me a lot in the early games when I was down to my last Pokemon with no healing items and halfway through a dungeon.
Honestly Swarm's not useless. Yeah not that much Pokemon know it, but it's like Overgrow, Torrent, and Blaze. And Illuminate only attracts more Pokemon. Normalize only gives STAB for the Normal type Pokemon, such as Skitty.
Well you gotta limit Slaking somehow, with those beastly stats.
And remember Regigigas' Slow Start and Archeops' Defeatist are there to also stop them from being too overpowered.
But I do have to agree that Normalize is useless, since it removes super-effective attacks and the only gimmick with it is to afflict Ground-types with Electric attacks. No wonder I only use Skitty with Cute Charm.
Truant is actual a very good ability when paired with a Pokemon that can swap Abilities around, most notably with Dream World Durant's Entrainment.
Normalize is probably the most useless ability because nearly every Pokemon can learn Return.
Another ability I hate seeing on a good Pokemon is Damp. Both Kingdra and Jellicent get it as their hidden abilities. :[ Their other abilities are so good it's like why even bother getting a DW Horsea? And there is a stationary Damp Jellicent in BW2...such a waste!