Well, here's my reasoning on various things:
Even x4 SR weak isn't world-ending:
It's far from ideal, but if the type provides many other benifits, SR is easy enough to prevent in Gen 5, and SR isn't even as common as in Gen 4 anyway. Volcarona is x4 SR weak, but you still see it around. Moltres is x4 SR weak, but dominated UU in Gen 4. Ho-Oh is x4 SR weak, but is Uber.
The type should not reduce the resistance to Fighting
Between Conkeldurr, Breloom, Tonadus, Thunderous, Machamp, Blaziken, Virizion, Lucario, Terrakion, and more, fighting type moves are all over the place. The fighting resistance HAS to stay.
This rules out Steel, Dark, Normal, Rock, and Ice
The type should not try to re-do something that is already done:
Looking at you, Gliscor, you're pretty good at momentum as it is
Ground is out, we'll just wind up re-making Gliscor, or a specially based Gliscor
The secondary type should not comound weakness to Water or Dragon, if possible, it should provide a resistance, or a way to combat them
These are also common attacking types. We don't want a weakness to them. lease note, there are two abilities which grant immunity to Water.
Dragon is out, Fire is Out
This, by my reconing, leaves:
Bug, Electric, Fighting, Ghost, Grass, Poision, Water and Psychic, as well as Pure Flying, which will probobly deny Roost.
Bug:
Pros:
x4 Resistance to Fighting
STAB U-Turn
Generally gets useful status moves
Cons:
WAY overused type [Although, no good pokemon have it]
Weaknesses everywhere
x4 SR weak
Overall: Probobly safe to rule Bug out
Electric:
Pros:
STAB is Super Effective on Water-Types
Volt Change
Thunder Wave
Adds NO weaknesses, cancels out weakness to Electric, adds Steel and Flying Resistance.
Cons:
Ground-types can switch in free, includeing on Volt Change, stopping mometum
Little to hamper Dragons
Zapdos exists, as does Thunderous
Overveiw: Good on paper, but, Zapdos and Thunderous both already do this job rather well...
Fighting:
Pros:
Fighting is a good STAB, and has excellent coverage with Flying
Provides SR Neutrality, and Rock neutrality
Cons:
Provides no resistances that are really useful in the current metagame, except Rock
Provides a weakness to Psychic
Fighting has little in the way of Momentum moves, besides possibly Overhead Throw for Phazeing.
Overveiw: Good for offensive based-momentum, bad if want defense
Ghost:
Pros:
Provides Immunity to Fighting and Normal
Decent STAB
Some decent support moves
Cons:
Gengar exists, he already does Mometum rather well, same with Dusclops, to a degree
Not very useful resistances brought to the table, Bug and Posion?
Ghost and Dark weakness gained, both somewhat common attacking types, as coverage moves on Special attackers [And Tyranitar]
Overveiw: Solid choice all around, but faces still competition from Gengar for offense, and Dusclops for Defense.
Grass:
Pros:
Support, Support EVERYWHERE
Provides Water resistance
Provides Electric Neutrality
Cons:
Dreadful STAB
x4 Ice Weak, Fire weak, Flying Weak, LOLPOISION weak, Bug Neutral
Overveiw: So-So, IMO. The merits of support, such as status and Aromatheraphy, are heavily contested by the cons...
Poision:
x4 Fighting Resistance, x4 Bug resist, x4 Grass Resist, x2 Poision Resist
Particually good for defense
Poision-types generally get somewhat wide movepools
Toxic Spikes?
Cons:
Worst STAB in the game [Although, not *that* bad for neutral coverage ATM]
Psychic weak
Overveiw: If we want to take a defensive route, Poision may be worth a look, otherwise... no. Even with defense... Renkulus exists, and hits us SE. Yay?
Water:
Pros:
Brings resistances to Fire, Water, Steel, and Ice Neutrality.
Good STAB
Some semi-support options, in things like Scald
Generally gets Ice moves too
Cons:
x4 Electric weakness, Grass Neutrality
Pretty useless for defense, just look at Mantine, he has the stats to be a special Skarmory!
Psychic:
Pros:x4 Fighting Resistance
Psychic Resistance
Some decent moves [Trick, Power Swap ect could be useful for trolling set-up pokemon, Trick Room]
Cons:
Dark Weak
So-So STAB
Bug Neutrality
Ghost Weak
Psychic/Flying isn't winning awards for coverage
Overveiw: Has potential for a defensive CAP1. Offense, not so much.
Summary:
What secondary type I feel we should take depends on the direction we want to take this CAP.
Offense:
It's a punch-up between Ghost and Fighting here. Water and Electric make strong cases too.
Defense:
My personal favorite for defense is actually Psychic, due to Trick, Power Swap, Trick Room, ect. Poision and Water are also good options.
Balanced:
A balanced CAP1 could best do with Psychic, Water, Electric, or Grass typings.
What do I think we should do?
I think we should be taking CAP1 down a balanced route, so that it can effect momentum in a way to suit the team it is on. For this reason, I think that Water, Psychic, or Electric should be chosen. Water if we want a true 'Jack of all Trades', Psychic if we want a Defensive lean, and Electric for an offensive one.
Also, looking into this thread again, I think an afterword is needed:
PSYCHIC IS NOT A BAD ATTACKING TYPE ANYMORE. If you think Psychic is a bad attacking type, due to it being resisted by Steel and Psychic, and Darks having an immunity, bear in mind that Flying, Psychic and Bug resist Fighting, and Ghosts are immune. Fighting is generally seen as a great attacking type. MORE POKEMON CAN SWITCH ON THE 'GREAT' FIGHTING COMPARED TO 'LOLPSYCHIC'