A problem I see is that the big teams will say that they are unable to 'downsize' rapidly. What I therefore would like would be an interim regulation over the time span of 5 or so years which would give big teams the necessary time to structurally downsize.
Let's say that you'd have an 'unlimited' class, which is what F1 is now: strict rules, but spend what you have.
Then you'd obviously have the 'capped' class, which has to work under a budget cap but with a lot of freedom on how to spend the limited resources.
Between that, you'd have 2 or 3 other steps, where teams can have, say, 125%, 150% and 175% of whatever will be the defined budget cap. They get a little more money, in order to keep their staff and infrastructure afloat for a few years, but are not as free regulation-wise as the completely capped teams. This could work like tokens in the power units right now: every part of the rules (for the whole car, not simply the PU) is given a certain value, with parts of the car that are more influential on performance than others being more "expensive" to buy (with a front wing taking for example 5 tokens and brake ducts 1, or whatever). Teams could therefore slowly go down in spending, and getting more engineering freedom each year. It would be a balancing nightmare, but it might be a feasible way to get everybody in line.
While Sauber, Force India and so on could go under the cap directly, other teams could start spending a little less and choose in which part of the regulations they want to get more freedom. Mercedes, I assume, has lots of staff, and so they could start in the 2nd class from top, use 175% of the budget that is defined as the budget cap, but only get a very limited number of tokens. If they think they might get a performance advantage in the PU department, they could spend their tokens there and start to have some PU freedom. Red Bull, on the other hand, might take the same 175% class, but choose to buy itself more freedom in aerodynamics using their tokens.
Obviously, the incentives should be for everybody to downsize as soon as possible. It even might spice up things: imagine Force India or Sauber to come up with some sort of holy grail, such as a blown diffusor or the F-duct, and the big teams cannot copy it, because they have put their tokens and development freedom into the rear wing and suspension!