Just a couple of notes:
1. There is a 116 page legislative proposal which was released by Sen. Wyden and other co-sponsors which actually does provide the details behind this idea.
2. The annual mark-to-market rule and resulting tax applies to “Tradeable Covered Assets” which include publicly traded assets, assets readily traded in secondary markets (historically this definition has captured certain derivatives and non-publicly traded debt instruments), assets traded on online marketplaces which match buyers/sellers (crypto), and assets for which the Treasury Secretary determines a reasonable basis exists to annually value an asset. Obviously this last category is the key item for the Koch family, etc.
3. Non-tradeable Assets are covered by a separate provision in the bill. These assets aren’t subject to the annual mark-to-market rules and gain will be taxed when the asset is sold. However, there is an interest charge applied to this tax to account for the “deferral” period of the gain. Tax and the interest charge is limited to a maximum of 49% on any gain. This provision is generally intended to cover privately-owned businesses.