Another potential professional partner is your prototype designer. Again, like the patent attorney and patent agent, this is something that is suggested to prototype designers with some regularity. Unless your invention is that one-in-a-million idea, your prototyper would probably prefer to be paid in money than to bet on the success of your invention.
Almost any professional whose services you will use in the development and marketing of your invention has the possibility of becoming a professional partner. If you choose to pursue this avenue, be cautious not to sign on too many professional partners. This is another area where you could find yourself in the position of becoming a minority investor/owner of your own invention.