I'd expect to be running tens of experiments at any one time. Some of those experiments might be variations in wording or colorschemes - others might be entirely different signup flows.

I'd let the experiment framework decide (ie. optimize) who gets shown what.

Over time, the maintenance burden of tens of experiments (and every possible user being in any combination of experiments) would exceed the benefits, so then I'd want to end some experiments, keeping just whatever variant performs best. And I'd be making new experiments with new ideas.