It takes two to marry but only one to divorce. If your spouse wants out, you save time and tears by accepting the decision and attempting to resolve financial issues amicably. Although a future inheritance probably doesn't constitute marital property in your state, you and your spouse are free to mention it in an equitable divorce agreement.
Some states, like California, divide property among spouses under community property laws, which attribute everything earned by either spouse during a marriage to the two spouses equally. An inheritance or gift is the separate property of the spouse receiving it. In other states, marital property includes all property owned by each spouse at the end of a marriage. In either case, an inheritance one spouse expects to receive in the future is generally not considered when dividing the couple's marital property.
However, when a couple prepares a marital settlement agreement, no law prevents them from mentioning a future inheritance. For example, if the custodial spouse is not seeking child support because she expects to inherit a fortune in the near future, this reasoning belongs in the agreement. Alternatively, if one spouse agrees to accept less than his fair share of marital holdings because of a future inheritance, it is appropriate to mention that fact in the agreement. Knowledge of the future inheritance may convince a court that a one-sided settlement is, in the long run, equitable.