Increase in Centralization: One of the primary objectives of the Fediverse is to maintain a decentralized network that is controlled by its users, not a single entity. However, the participation of a large corporation could potentially lead to an increased centralization, undermining the very principle of the structure.
Monetization and Profit-Driven Activities: Large corporations are generally aimed at generating profit. They could try to introduce monetization features which can change the way the Fediverse currently operates, moving away from the principle of a free and open internet.
Data Privacy and Security: Large corporations sometimes engage in data mining for targeted advertising or selling information to third-parties. Their participation could raise serious concerns about data privacy and security within the Fediverse.
Influence Over Standards and Protocols: If a large corporation becomes a dominant player in the Fediverse, they might impose their own standards and protocols, or make alterations to the existing ones.
Culture Shift: The Fediverse is largely driven by a community that values internet freedom, privacy, and decentralization. A large corporation could change the culture and nature of interactions within the Fediverse.

Bear in mind that these are potential issues and not guaranteed outcomes. The unique structure of the Fediverse itself can provide some level of resilience against these concerns.