I feel like the distinction between copyright (with term limits, but strong protections) and trademark (with no term limits, but strong weaknesses) should be enough for a company like Disney. Maybe there should be some sort of "franchise copyright" somewhere in between the two, but I've never heard a good specific idea to that.

However, I heard one proposed idea that prolonged copyright extensions (or variable copyright terms, if you'd prefer) might make sense as a corporate property tax that increases over time after the baseline expiration (and better if you reset the baseline to something smaller again).

That would at least trigger bottom line decisions in corporations if keeping something in copyright (and out of the public domain) is worth the annual property taxes on it. Some of that tax could go directly to archival/preservation efforts for those properties in the proper spirit of insuring their legacy for the public domain, eventually. Most of that tax would indirectly go towards discouraging companies from IP "tenements" where rent is collected (subscription/access charges), but innovation/investment diminished a long time ago. (Mickey has new cartoons on YouTube every so often, but how many properties does Disney own that other than maybe a bare conversion to Netflix or soon Disney+ streaming they haven't done anything with in decades?)