I've commented multiple times about hydraulic hybrids. I like them, relative to electric hybrids, because they have a very high power density. I like the acceleration that power brings. And 1,000 charge/discharge cycles is hard on batteries but pretty much a normal day for hydraulics.
The amount of energy stored in a hydraulic accumulator is based on the volume and the square of the pressure. Ergo, a 5k psi system has (5/3)^2, or approx. 2.7x, the energy content of a similar volume accumulator running at 3k psi. So, higher pressures should be embraced, as much as possible.
Power is the combination of pressure and fluid flow. If you're going to have a vehicle powered by hydraulics (as Karl and I had discussed, oh so long ago), you want maximum pressure and minimum flow (in much the same way that an electric vehicle tends to be more efficient if you use high voltage and minimal current). This means that, if you're going to have hydraulic accumulators buffering the surges in input/output, you probably want to keep the power train running at, say, 5k psi and have something resembling a Constant Speed Drive (in this case, a Constant Pressure Drive) modulating the accumulators' pressure up to/down from the system pressure. That way, the accumulators can vary in pressure up/down the scale. Since kinetic energy goes up with the square of speed and stored energy goes up with the square of pressure, a simple pressure gauge on the accumulators would give some idea of relative max speed achievable with current stored energy before the vehicle devolves into "gutless wonder" mode.
A device which would work well for the Constant Pressure Drive is known as a "hydristor," which is supposed to be a portmanteau of "hydraulic" and "transistor." It's a twin-chamber, variable geometry, vane-type hydraulic pump/motor. You can have hydraulic fluid of one pressure driving motor-style in one chamber and hydraulic fluid of a different pressure driven pump-style in the other chamber. Ergo, 5 gpm at 500 psi (from the accumulators) can produce approximately 1/2 gpm at 5k psi (not 100% efficient, but rumored to be pretty close). You can use the shaft in the middle of the whole thing as a mechanical power input/output or you can just do hydraulic power conversion and ignore the shaft. Unfortunately, the guy holding the patent on that has died and his patents are kinda up-in-the-air, at last check.
As a bare minimum, I'd like a compact vehicle with the transmission pulled and the main hydraulic pump/motor attached to the differential gearing. Use a small gasoline or diesel engine to provide "cruise power," with the engine driving the main hydraulic pump. Let the accumulators/CPD operate as a "L1 power cache" (referenced in one of my earlier Journal entries), so that the engine stays at a pretty steady throttle setting. It would throttle up/down, depending on demands and the current state of the accumulators (heavy demand and rapidly falling accumulator pressure = more throttle, etc.).
Eventually put an electric motor/generator and batteries into the mix. Obviously, electricity from the batteries can turn the electric motor, turning a hydraulic pump to drive the vehicle. Hydraulic pressure from the engine can turn the generator to charge the batteries or pressure from the electric motor can "roll start" the engine when needed. This would allow the batteries to function as a "L2 power cache." Eventually, add enough batteries that your typical daily drive uses no fuel but you still have the option if you need to travel further than usual.
The only remaining questions: how heavy (5k psi system would probably be lighter than 3k, as less fluid is needed) and how expensive?