Before I add my input, I want to tell people to tone down on the custom ability suggestions. This is a concept that provides plenty of existing options for its implementation, and speaking as somebody who has suggested a custom ability that got implemented, these sorts of things tend to get very contrived. If we already have the tools for the job, there's no point in making more of them. The likelihood of me allowing any of these custom abilities into the final poll is quite rare.
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This post will detail abilities I like and don't like, based on my own vision and the discussion I've seen in this thread. If the ability is bolded in black, it means that I think it's workable. If it's bolded in red, it means I'm either somewhat or highly opposed to allowing it in. If it's not bolded at all, it means I'm undecided and would appreciate additional discussion on the specific ability. Basically, these are my opinions at the moment of me viewing this thread, and I encourage people to make cases for and against them
Trace - I really like Trace for this Pokemon. As Deck Knight noted, there is a decent number of Pokemon that are directly affected by Trace upon you switching into them, but there is also a decent number of Pokemon that aren't affected at all. For this reason, Trace is a great ability for covering a number of threats, while leaving a number of Pokemon uncovered for another ability's help.
Intimidate - While Trace is a pretty general ability that can be effective against a number of Pokemon, Intimidate is a much more specific choice, but I still like it a lot. As it's been said, this Pokemon is ultimately going to have the most trouble taking physical attacks, due to its decent resists on the special end. Whereas Trace works to Intimidate Gyarados and Salamence, the actual ability Intimidate allows you to cover many more threats in the same way that otherwise might bypass you, such as Tyranitar and Lucario. All in all, I think this is a very solid choice.
Magic Guard - I'm not as outright supportive of Magic Guard as I am of Intimidate and Trace, for the reason that this ability doesn't actually pertain to countering a lot of specific Pokemon on an offensive level, but it should be noted that this Pokemon isn't strong enough to deal with a lot of Pokemon that rely on status to beat their usual counters. For example, we may resist Fire, but due to a combination of the Ground weakness and the vulnerability to status, Heatran may potentially do a LOT of damage to us with a Sub Toxic set, even if we do manage to beat it in the end. This also pertains to other semi-offensive status users like Zapdos and Rotom-h. Also, being able to switch into Spikes and SR without a problem means that you can effectively switch into more threats without an issue. For this reason, I think Magic Guard is an acceptable choice.
Filter/Solid Rock - In addition to strong neutral attacks like Lucario's Close Combat, this Pokemon is going to have some difficulty taking strong Earthquakes, and other than Celebi's and Infernape's Grass Knots, the majority of the Grass-type attacks that are sometimes seen in OU are going to do a lot of damage. For this reason, an ability that provides a direct answer to both of these problems at once is a valid choice IMO. This particularly goes for the Grass-type weakness, because while most Earthquakes are not backed up by STAB and can potentially be tanked without this, our Pokemon is going to have an immense amount of trouble surviving any Leaf Storm, or taking a Seed Bomb from Breloom. This fact alone makes Filter/Solid Rock something to consider.
Special Intimidate - This was originally a choice that interested me to an extent, but after considering it, I've realized it's neither a wise or a practical choice for this CAP. Let's look at it this way. Electric/Water really doesn't have a lot of problems on the special side. We are already resistant to three of the most powerful and common special attacking types, Fire, Water and Ice, and we take neutral damage from Electric. The only special attack that is going to be difficult to take twice in a row is STAB Draco Meteor from 110 base SpA (I'm aware of the Specs Jolteon issue, and feel that with the spreads in the running, it's very possible to EV this Pokemon to respond to Jolteon and other Electric types). There are only two of these. We are faster than Salamence no matter which spread we pick, meaning that we will only need to be able to take ONE Draco Meteor to force it out, and either Ice Beam it or use that turn to heal or initiate another strategy. This means that the only issue is Specs Latias (because we will generally be okay against other Latias), and both EV spreads can be EVed to take two Draco Meteors in a row from it. For these reasons, I find Special Intimidate to be an impractical, superfluous custom ability that doesn't bring anything to the table.
Levitate - At a glance, this actually looks like a good ability to be using for this Pokemon, but I really feel that using this, or any other immunity-inducing ability is kind of a cop-out, because the only weakness that this would be addressing, Ground, is really not the worst thing in the world to have and can be indirectly remedied by other abilities that bring more to the table all-around. Intimidate weakens every Earthquake user to a point where pretty much no Earthquake in the game can OHKO our Pokemon, Trace can potentially copy any other immunity-inducing ability in the game depending on what you switch into (including Levitate against Flygon), and Filter/Solid Rock can reduce both your Ground weakness and your Grass weakness at once. For these reasons, I don't see why it would be practical to allow Levitate to eat up an ability slot.
Shadow Tag - This is not happening. CAP10 will have decent defenses, a lot of HP and a high Speed stat no matter what spread we pick. Wobbuffet basically has these traits, but is slower. I have no idea why people have this idea in their head that we could possibly make a Pokemon that isn't broken using this ability. It isn't going into the poll. Period.
Download - I'm pretty ambivalent on this one. On the one hand, being able to adjust your offensive stats to better hit certain threats is actually a very interesting way to fulfill the concept. Being able to boost your Attack or SpA could be invaluable against bulkier offensive threats like Latias, Tyranitar and Machamp. On the other hand, Download could potentially take this Pokemon in a somewhat offensive direction due to the high Speed and decent offenses CAP10 is guaranteed to have. It's also a bit troubling that the boosts don't go away after the threat in question switches out. For these reasons, I could definitely swing either way with this ability, and I'm very interested in seeing increased discussion of its validity to help me render a decision.
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As I said in the beginning, feel free to critique and argue with my opinions. Basically, my goal is to be able to reach a consensus on what is going to the poll by the time this thread finishes, so any and all discussion is going to be making this easier. This thread will be closed sometime tomorrow afternoon or evening, meaning that you have additional time to make your points.