I don't mean conversion to Christianity (or Judaism, or Hindusim, or . . .), but conversion to (lower-case) democracy. They must accept that infidels have all the same rights Muslims do. Since this would be a violation of the Koran, I describe it as "conversion," which is essentially the same term the Muslims use.
If they won't convert, then Dhimmitude is the next alternative, just as the Muslims offer it to Christians and Jews in Muslim countries. We need to rule the Islamic world in much the same way the British Empire ruled India. Let them have those customs that are harmless to us, or that don't violate basic human rights (like suttee), but otherwise just keep the lid on them.
If neither of those work, there is only one alternative left. And we'd better make it clear we mean it.
You really mean republicanism, not democracy. In a republic we recognize a higher authority than the State. (Our brand of repubicanism grows out of Christian freedom, but that is another discussion.)
Other than this criticism, this approach seems the only secular answer that has some chance of working.
Acutally every Muslim other than a tiny percentage are descendents of Jews, Christians, Hindus, Bhuddhists, Parsees, Jains, animists (did I leave anyone out?) who were forcibly impressed into Islam. Some only a few hundred years ago. They should all be given the opportunity to convert back to the religion of their forebears.
Of course, that is punishable by death in Islam. But who knows how many would take that choice if they were free to do so?