Ghost/Dark Yamask
Well, Knock off is everywhere, and Yamask has trouble dealing with fighting types as a result. This typing would not only cause it to lose its weakness to dark, but also make it more effective at dealing with other ghost types such as Misdreavus. Its typing would as give it immunity to Meditites Stabs and force it out with mummy removing Pure Power.
my notes: gg meditite, and every other fighting-type, this thing can do a lot, including trick room, will-o-wisp, toxic spikes, this thing even gets knock off, but it has an off-putting 30 base attack, which I don't think will be that much of a problem giving the stab and usefulness of the move.
Recover Koffing
I think Koffing missing reliable recovery is what holds it back from being a great LC Pokémon. He can tank physical hits all day long, but he eventually runs out of health. Sometimes, having just one or two extra turns to spread spikes and status could make a big difference.
my notes: great fighting-type wall, it's problem largely has been simply running out of health, and he gets toxic spikes now, so he can do even more for defensive teams
Recover Grimer
Grimer's biggest flaw has always been a lack of solid recovery. If it would have Recover (which, flavor-wise, kinda makes sense), it would be the ultimate Fighting/Fairy (read:spritzee) killer.
Mienfoo's Fake out/U-turn/Stab all don't doo too much damage, while Knock Off wouldn't knock off Grimer's item (let's assume Eviolite) because of Sticky Hold. It doesn't care too much about Stone Edge/Hi Jump Kick either. Taunt is somewhat annoying because it blocks Recover, but there are plenty of other mons Grimer can recover on.
Recover Grimer would also allow it to run something else than Sticky Hold, since it can afford to lose it's Eviolite. Poison Touch is interesting, since if you combine this ability with the 30% poison rate of Gunk Shot, you'll have a 51% chance of poisoning the target (if gunk shot hits)
Recover Grimer would just be pretty amazing overall.
my notes: I think this would give an even bigger reason to run sticky hold, since it can constantly block knock off users, and it fairs pretty well against a lot of them. This thing would function as a tank as opposed Koffing's defensive role, and has better all-around bulk, thanks to it's higher HP stat.
Magic Bounce Nosepass
It should be noted that the only mons gifted with the ability Magic Bounce are also weak to Dark And Ghost, types which offer a vast array of mons with many tricky status moves. What makes this bad is that the Dark and Ghost mons can still overpower the Magic Bounce carriers, hitting them for super effective damage with their STAB attacks, which really makes Magic Bounce partly a waste as far as its distribution. The raw point of this in relevance to the LC tier is that Natu CAN block Krow's Confuse Ray and T-wave, and Missy's WoW, but they don't give a damn because they can mow Natu down with force anyway. With the sheer high usage of such mons in the tier, and as good as they are, this makes Magic Bounce far less appealing than its effects entail.
But what if a mon who wasn't weak to those types could get Magic Bounce? Imagine Prankster Krow vs Magic Bounce Nosepass, or really ANYTHING who could break this typical pattern of Magic Bouncers being defensively weak to so many of the common tricksters. To me, this is a very happy thought, and I believe that such an addition could be fruitful to the LC metagame and could bring Magic Bounce's true worth to the forefront.
my notes: one big thing about this, is that with gligar gone, it's arguably more difficult to keep hazards off the field, a more viable magic bouncer helps alleviate this, and it has a cool little movepool, with stuff like volt switch, stealth rock, and thunder wave.