I'll present a series of intuitive definitions, and exhibit the problems with each:
(1) A motion is fair if teams in every position have an equal chance of winning.Of course, this may not hold between teams of wildly differing skill levels, so we'd better modify this to:
(2) A motion is fair if teams of equal skill level in every position have an equal chance of winning.Using definition (2), or indeed, anything like it, suggests a problem that should be familiar to everyone who has set motions for a debating competition. Motions can only be fair or unfair relative to the set of debaters expected to be debating them. Anecdotal evidence suggests that many conceptually advanced motions (e.g. THBT the state should pay reparations to women.) may give all teams an equal shot at winning when all the teams in question are excellent debaters. However, those same motions may be much harder for one side when debated between novices. In some cases, this is because of a conceptual bar that teams must leap before being able to engage in the debate proper. In other cases, it's because a certain level of rhetorical faculty is simply necessary to make some kinds of arguments convincing. In any case, this is a problem not just with definition (2), but with any definition of fairness that is tied to outcomes between teams of equal skill.