Tenants understand the need for step-ups in their leases because they recognize the inevitable upward trend of costs. There is no reason for elaborate set of definitions in the lease. The lease simply needs to spell out in a schedule the applicable period and the rent to be paid during that period.
The landlord will almost never agree to a step-down rental tied into the possible deflation rather than inflation. However, there is one instance where the demand of the tenant for a step-down in the rental might be appropriate.
When a landlord has been able to get good mortgage financing for the improvement primarily because of the high credit rating of the tenant, that tenant may argue that when the initial mortgage is fully paid off, the rental should be reduced. The cash flow to the landlord will jump substantially because the debt service is eliminated and the tenant may want a share. In effect, the tenant says that since his credit standing made the mortgage possible (and consequently created desirable leverage for the owner), the tenant should benefit as well as the owner when the mortgage is paid.