What about something along the lines of Breloom's poison heal? This is kinda silly but for example something called "steam powered" might raise speed when burned, or maybe just when hit by a fire move including will-o-wisp.
I like this abillty or anything else that would give it alot more switchins, like things that gives immunities. Watermons types has good resisdence, but doesn't give it any immunity. Immunities would probobly be nessecery for this pokemon to be able to switch in alot. You can't deny that immunities is an uninportant thing when 9/10 of the top could have an immunity (counting Strats Levitate and Syclants Mountaineer because of the switch in oppertunity. But still 7/10 got immunities only by type). Immunities makes you able to come in without any dammage and is really important for making good team-synergy.
An ablility similar to
dry skin would probobly be great. Because it gives it an immunity but balances up with an extra weakness. I could se this pokemon get an ablility that makes it
immune to Dark attacks (thinking mostly of those fishes living deep down under the water without any sunlight at all) and gives it a Poison weakness, it could be called
"Pristine Darkness". The poison weakness would probobly not be so big, because of the rarity of Poison attacks (mostly due to all steel types in OU). Of the top 50 of the most used pkmn on the
CAP server during november only three pokemon is of poison type: Fidgit, Gengar and Tentacruel, and I haven't seen any of them use Sludge bomb alot. I could see it
get two immunities and become 4x weak to Poison as it would make it more counterable (probobly with alot more not OU pokes) and a good decentralizer of the current metagame.
If this pokemon should "decentralize" the metagame, it needs to counter (or "check" or what you else call it) the top10 so good that you'll need something else to get around it. But it still needs to be weak to alot of other pokes (not of the top 10), just so it doesn't just becomes 1 of the new top10. That is the goal of CAP6 as I see it.