The other Roddenberry pilot that got produced in his lifetime was the supernatural-themed Spectre, which I've never seen, so I can't speculate on how it would've worked as a series.
I remember forcing myself to sit through SPECTRE and finding it hard to believe that with such a great couple of principalcast members, it could be so flat and almost unwatchable.
I think QUESTOR shot its wad for a different reason than having already resolved Questor's nature (his incompleteness actually does work to make the characters complementary if it went to series); the series wouldn't have had John Vernon's character, which is what gave the thing its genuine complexity and depth (I've always thought that character was nearly entirely Coon's.)
Having bought QUESTOR on DVD recently, I was pleasantly surprised to find that the Vernon character is just as good as I remembered (I pretty much remembered the dialog at the end verbatim, even though it had been decades.)
I suppose that they could have had a similar character in the series, but I can't imagine him having the ethical weight. I absolutely see Vernon's perspective about man, and I always tear up when he takes the homing device (have watched the last 10 minutes about 5 times already since buying it.)