I just started a new physical business, and this is what I shared with the partners; I informed them about my intended exit plan with gradual transfer of my responsibilities via new hires and training them. I also clarified my position after exit that my exit from business operations does not indicate in lower future benefits.
After sharing this, Ask if they have anything like that in mind.
Other matters that needs to be discussed includes, clarification of responsibilities, the vesting period, agreement for the case of dispute and who will own what, etc. make sure to get it documented legally(I made a mistake here for said business and I am regretting it).
Best advice I ever got was from our lawyer during the formation of our C-corp when we outgrew our partnership: "It's important to decide how you will split up now, while you are still friends, because when the time comes to split up, you may not agree." So true.
While I give him credit for this advice, lawyers in general are overpaid and not usually very reliable in my experience. This same lawyer gave us our corporate papers printed with some other company's details, including their bank account information.
Don't put too much trust in a lawyer unless you have a lot of experience with them to back up that trust.
The reason I think this is important, is the start is always a grind. But, when you get to the stage of going full-time, worst feeling is when one side thinks they are contributing more. Being aligned on expectations from the start will save lots of headaches in the future.
Another question, which I think you need to ask yourself, is if you would enjoy working with that person. Going with a cofounder means you're going to be spending a lot of time together. They could be the best in their field, but if they are insufferable, it's not worth the headache.
Money is the single most important factor that breaks startups just as they're about to take off. Better to clear it up at the beginning.