How can I plan a VFR flight through controlled airspace, without knowing what instructions ATC will give me?
Short answer: make your flight plan as usual but if something changes then just deal with it. After all, as a private (?) pilot you've been trained to handle that situation.

Even an IFR flight plan is only 'plan A' and on any flight you may have to deal with changes because of ATC, weather or other issues. That's one reason why the private ACS includes a diversion task to test whether you can re-plan in the air. Note the wording of that task (emphasis mine):


  Make a reasonable estimate of heading, groundspeed, arrival time, and fuel consumption to the divert airport.


It's helpful to think about general options in advance (e.g. "I'll go west around the KPHL class B if I don't get a clearance"), but you can't predict every possible situation. Always having that "reasonable estimate" for where you're going is more important than trying to anticipate and plan for every eventuality.

The good news is that as part of your original flight planning you should already have reviewed all the information you need to make decisions in the air: winds aloft, fronts, airspace, TFRs etc. So if you do have to re-plan you already have a good overview of the big picture and what your options are. "Plans are useless, but planning is essential", as someone once said.

And if you don't already have a flight app like Foreflight or Garmin Pilot then you should look into it. They make planning, diversions and replanning much simpler, which removes a lot of stress from the situation. They're also much easier to work with than paper and pencil, especially if you're single-pilot without an autopilot.

Finally, don't forget the magic word "unable". ATC can't see what you see, and they might drop you in an awkward situation, e.g. you're VFR between cloud layers and the only way to stay in VMC is to enter class B. If that happens, don't hesitate to tell them that you need a clearance, or vectors, or whatever. If they don't cooperate, you can always declare an emergency if the situation is becoming unsafe (but don't abuse the system for your own convenience).