Personally I think most abilities that don't outright launch an attack against a target or are turret/drone based should have 3 modes:
Initiated - One use per cooldown, the cooldown is the amount of time the power core requires to recharge the ability. This is the most powerful version of an ability.
Very basic example: Power Boost, provides 10% bonus damage for 15 seconds, costs 300 power to recharge
Togglable - Players can activate or deactivate the ability at will, but while active there will always be a constant power drain.
Very basic exmple: Power Boost, provides 7% bonus damage @25 power per second while active.
Note by design that togglables will slow down Initiated cool downs.
Passive - The ability is always on and doesn't require any extra power beyond minimum requirements, but provides the smallest boost.
Very basic example: Power Boost, provides a constant 4% bonus damage.
(obviously you can only have one version of an ability, IE you can't install all 3 versions of an ability on your frame.)
In terms of design and fine tuning, we need to strike a balance between how unique we can make our omni-frames (which is important) and preventing any one player from "being able to do everything". Passives are important because it allows a frame to be diverse, without being overpowered and also keeps the actual ability buttons to a minimum. Obviously the number of slots is going to come down to game design and power balancing, I suspect that anything from 2 full/2 passive to 3full/3passive is going to be good, less than that restricts customization too much, more than that could give a player too much individual power. I suspect that if abilities are too watered down to justify more slots than that, the abilities themselves won't be very interesting to use outside of flavor. But again this is all opinion, I am not a game developer.