Add a proration field on every inventory item! This would be a key feature for any sponsorship team. The use case would be for any inventory sold for a portion less than a year. For example if all assets in a deal except for one are for the full year, and that one item will only run for 3 months, there is no way to represent that in your data without having to create that item into a seasonal AND individual rate card (which makes your inventory very messy) and then tying it to an event list.
This could be as simple as having a field where you could put a percentage or fraction for how much you want to prorate the rate card value by for seasonal rate cards. This would be especially useful for organizations that lock their rate card values !!
Overall, this would result in better user experience, inventory management, and most importantly accurate data.