What is the limitation on what records a .com domain can have?

When you register a domain name, what that gets you in technical terms is the delegation of a zone by that name to the nameservers of your choice.

What that delegation means is that the parent zone adds NS records for your zone and, if needed, glue address records (A/AAAA) records for the names in the NS records (as well as DS if the child zone is signed). The TTL of these records follow whatever policy the registry has in place.
These NS and glue A/AAAA records are not authoritative and are only used to locate the authoritative nameservers that can answer queries for names that are part of the child zone; ie, these delegation records are not used to answer client queries even for these NS or A/AAAA records.

Delegation of zones is how registries operate in general, this is not specific to com.

What you are asking for sounds like an alternative centralized model where the registrant does not get a zone of their own that they can operate as they see fit, instead having the registry host the registrants records as part of the TLD zone.
That is not the model that registries use, but it would be a technically possible alternative kind of service to provide.