I think forcing Fire-types to run Will-O-Wisp and thereby diminishing their offensive presence is one of the goals of this CAP, and for this reason I can't support Water Veil.
I also think a Water Immunity is trying to compensate for something that just can't be reasonably fixed. Of the two Water types we're targeting, a sufficiently offensive CAP (and definitely a Mega) can 2HKO with Gunk Shot or, since we're in the ability phase, how about we go for Rock Head to benefit Head Smash? This wrecks Manaphy and while Keldeo resists, a follow-up Gunk Shot with the necessary stats to KO would work splendidly. A resisted Head Smash has the same power as a neutral Rock Slide. Water-types could also be addressed with Grass coverage, a coverage which doesn't really affect any Ground-types outside Hippowdon (Lando-T, Chompy, Excadrill, and Gliscor are neutral) [Also Rock Head Wood Hammer Mmmmm].
"What do we want CAP21's Primary Ability to accomplish?"
In any case, I think our primary ability should match the concept and play to the types strengths and weaknesses and be relevant to Pokemon we want to threaten. Sand Stream does this perfectly, and undermines two of our targets directly, Volcarona and Zard-Y, that rely on special attacks to damage. After Zard-Y Mega Evolves it additionally takes its Sun away. Sand Stream also provides some scouting value for Leftovers, and can help identify Magic Guard vs. Unaware (70/30) Clefables. Additionally, this typing specifically is immune to Toxic and neutral to SR, and therefore difficult to wear down. It is also true that the most prevalent types it lures as checks (counters being physical Ground and Steel mons) use the Special Attacks Sandstorm buffers against. Sand Stream is more effective than Solid Rock at lowering Special Attack damage, and puts more target Pokemon into KO range. Lets also briefly mention neutrals. With Sand Stream and a good distribution, CAP has psuedo-resistance to Dragon, Fighting, Ghost, Grass, and Ice Special moves, and this can be leveraged extremely well because Rock/Poison STAB is neutral or SE on all of these.
As far as Base vs Mega emphasis, the ability to relaunch Sand Stream, if its worn out or been replaced is also a great strategic move, and I predict the enhanced stats will also make it highly desirable to be a Mega, but no so much so that you waste the strategic placement of using it. Even if we were to use a more offensive ability for the Mega and not have SS on the way back in without a partner like TTar or Hippowdon, the ability to capitalize on the 1.5x SpD would be beneficial.
Now, there is the question "What about Tail Glow" and, again, I know this is Ability but there is a move that wrecks Manaphy after a Tail Glow setup, and it's Topsy-Turvy. Manaphy's options on a faster TT mon is to either TG again to get back to 0, or use -3 Scald. Combined with Sand Stream this means you are building up damage, you can negate Rain, etc. Again, if CAP can 2HKO Manaphy with Gunk Shot you're trading that potentially for a burn chance, but nonetheless you do stop it in its tracks and provide something unique against a number of set-up sweepers. As far as defensive types go, Rock/Poison is pretty credible on switchin - Fire, Flying, and Fairy attacks are common offensive moves. All I'm saying is that we have credible options here, especially with the unique typing of this CAP.
I also think CAP can build bridges on Sand Teams not because it will replace Tyranitar but because it can be run along with it instead of Hippowdon. The secondary Poison typing provides a marvelous contrast for Tyranitar and soaks up a huge number of the attacks that target it. As a partner, nobody is going to complain about Tyranitar's own competitive abilities like in so many of the partner Pokemon discussions CAP has had in the past. In exchange, for the defensive cover Sub-Poison provides for Sub-Dark, there's nothing really bad about having multiple Pokemon with offensive Rock STAB on a team, since it hits so much for neutral or SE.
tl;dr I think Sand Stream most closely matches the concept of the CAP, provides a multitude of benefits specific to the Pokemon we want to counter, potentially forces them to run a support move instead of an offensive one, and provides the best plaform for a Mega-Pokemon by assuring that, barring certain forseeable conditions, assures that Mega CAP will be able to perform a utility counter role very successfully.
I also think a Water Immunity is trying to compensate for something that just can't be reasonably fixed. Of the two Water types we're targeting, a sufficiently offensive CAP (and definitely a Mega) can 2HKO with Gunk Shot or, since we're in the ability phase, how about we go for Rock Head to benefit Head Smash? This wrecks Manaphy and while Keldeo resists, a follow-up Gunk Shot with the necessary stats to KO would work splendidly. A resisted Head Smash has the same power as a neutral Rock Slide. Water-types could also be addressed with Grass coverage, a coverage which doesn't really affect any Ground-types outside Hippowdon (Lando-T, Chompy, Excadrill, and Gliscor are neutral) [Also Rock Head Wood Hammer Mmmmm].
"What do we want CAP21's Primary Ability to accomplish?"
In any case, I think our primary ability should match the concept and play to the types strengths and weaknesses and be relevant to Pokemon we want to threaten. Sand Stream does this perfectly, and undermines two of our targets directly, Volcarona and Zard-Y, that rely on special attacks to damage. After Zard-Y Mega Evolves it additionally takes its Sun away. Sand Stream also provides some scouting value for Leftovers, and can help identify Magic Guard vs. Unaware (70/30) Clefables. Additionally, this typing specifically is immune to Toxic and neutral to SR, and therefore difficult to wear down. It is also true that the most prevalent types it lures as checks (counters being physical Ground and Steel mons) use the Special Attacks Sandstorm buffers against. Sand Stream is more effective than Solid Rock at lowering Special Attack damage, and puts more target Pokemon into KO range. Lets also briefly mention neutrals. With Sand Stream and a good distribution, CAP has psuedo-resistance to Dragon, Fighting, Ghost, Grass, and Ice Special moves, and this can be leveraged extremely well because Rock/Poison STAB is neutral or SE on all of these.
As far as Base vs Mega emphasis, the ability to relaunch Sand Stream, if its worn out or been replaced is also a great strategic move, and I predict the enhanced stats will also make it highly desirable to be a Mega, but no so much so that you waste the strategic placement of using it. Even if we were to use a more offensive ability for the Mega and not have SS on the way back in without a partner like TTar or Hippowdon, the ability to capitalize on the 1.5x SpD would be beneficial.
Now, there is the question "What about Tail Glow" and, again, I know this is Ability but there is a move that wrecks Manaphy after a Tail Glow setup, and it's Topsy-Turvy. Manaphy's options on a faster TT mon is to either TG again to get back to 0, or use -3 Scald. Combined with Sand Stream this means you are building up damage, you can negate Rain, etc. Again, if CAP can 2HKO Manaphy with Gunk Shot you're trading that potentially for a burn chance, but nonetheless you do stop it in its tracks and provide something unique against a number of set-up sweepers. As far as defensive types go, Rock/Poison is pretty credible on switchin - Fire, Flying, and Fairy attacks are common offensive moves. All I'm saying is that we have credible options here, especially with the unique typing of this CAP.
I also think CAP can build bridges on Sand Teams not because it will replace Tyranitar but because it can be run along with it instead of Hippowdon. The secondary Poison typing provides a marvelous contrast for Tyranitar and soaks up a huge number of the attacks that target it. As a partner, nobody is going to complain about Tyranitar's own competitive abilities like in so many of the partner Pokemon discussions CAP has had in the past. In exchange, for the defensive cover Sub-Poison provides for Sub-Dark, there's nothing really bad about having multiple Pokemon with offensive Rock STAB on a team, since it hits so much for neutral or SE.
tl;dr I think Sand Stream most closely matches the concept of the CAP, provides a multitude of benefits specific to the Pokemon we want to counter, potentially forces them to run a support move instead of an offensive one, and provides the best plaform for a Mega-Pokemon by assuring that, barring certain forseeable conditions, assures that Mega CAP will be able to perform a utility counter role very successfully.