You can already do those things . . .
Set up a membership level "sponsors"
Set up a separate application/registration form
Set up separate renewal guidelines/notices
Do not give them access to members areas
We have these things set up and so far working well -
The thing I'm noticing is that we have members that would also like to become a sponsor and be able to place an "ad" on the sponsor list. Software does not currently let them use the same email address to do both. They can only be one or the other.
We have legitimate people that are members of the group and also have businesses that sell to the group - would be nice if they could use one email to access all accounts.