For undewater vehicles, there are only two systems for lifting power control, Dynamic and Static.
Dynamic systems provide power by vertical thrusting devices, or by control planes, as airplanes do.
The second one, is the simpler, but requires the vehicle to be moving at a minimum manouvering speed, that for a ROV is not very useful.
The first one, has been the choice for the OpenROV project, due to its building simplicity, easy control and low price, together with its good performance.
The fixed and releaseable weight, is a good option, albeit its a One Way only procedure.
Static systems use a change in the averaged vehicle density for buoyancy/weight balance control. That's the system actually used by Submarines.
Those systems, are the best for underwater operations, but are complex to be built. A good manteinence is required, and are expensive.
For big depths, variable displacement systems are not the most suitable. High counter pressure devices must be fitted in the vehicle in order to control ballasts, at the same time, very strong circuits (piping/valves ....) and tanks are required.
Wouldn't it be an intermediate solution carrying a lifting line, attached to the surface "unit" with the ROV, hooking the weight, releasing the line from the ROV, and heaving that line up from the surface ?
Making the ROV able to lift heavy loads, means a radical design change, but making it able to carry a lifting line, does not look so complex.