Technically it should already be possible for a company to advertise here, no? Not in the "there are little video boxes you can't get rid of (barring adblocker extensions)" but in the sense that one could have their employees create accounts and make comments and posts to promote their products. They'd probably have to do it subtly and sneakily, because they'd likely get banned or if they had their own instance, defederated, but they could. Wouldn't even need to pay anything beyond employee salaries to make it.

I feel like "proper" ads would be more difficult to implement, because even if the software were updated to include the ability to add them, people could and likely would make forks of it that just didn't display those from federated servers, or clients that don't on any server, and because the software is open source there would be no stopping it. An instance could defederate instances using such an ad-blocking fork, but that would risk ending up themselves isolated and therefore lose much of their traffic and viability as a platform.