The 555 is too slow for what you try to achieve (50ns increments), although pulse delay generators with such specs are commercially available, but that does not seem to be what you are searching for.
The DIY approach requires to understand that single pulses need to be quantified somehow to actually manipulate the output according to the requirements, which brings you deep into ADC territory.
Maybe you should point out which technology suits your application best, as this indicates how this quantization will need to be implemented.
It could be
- analog, then you have to know or somehow fix the impedance of your source
- digital logic/ASIC with proper propagation delays
- microcontroller fast enough for at least 20MHz output frequency, so when eyeballing the instruction cycles there should be enough margin to have the fighting chance to meet demands
Should you intent to use logic ICs for implementation you'd have to remind the propagation delay of the various gates (that stem from capacitance in every gate in the system; holding charge that needs to be dissipated first), which at 50-100ns becomes a significant part of the equation. A sync'd output signal might have to be fed faster than the logic operates, therefore you might need to "OR" the output with whatever delay circuit you come up with.
It is not a simple problem, actually it is a typical training question to get into these technologies.