A good co-founder will have at least 2 and ideally 3 of the following:
In addition all founders must have #5 Drive(/ability to execute/be willing & able to put the time and effort in).
Ideas are a dime a dozen. Here's the only idea you need: make something people want. If I were bringing just Talent into the mix I would take 30% if the other founder(s) were bringing in Money, Network, Ideas, and all had the drive. Also, here's a little secret: everyone ends up having plenty of ideas. In reality your successful mature business 10 years down the road is most likely going to look nothing like this person's initial "million dollar idea". If ideas are truly all that this person is bringing to the table then within 3 months you're going to have contributed enough, just on the idea front, to step back and realize that:
- You are contributing half the ideas and doing all of the work
- The important work that you are not able to do is either not being done or not being done well because there's more than ideating involved in starting a business
- You need to bring in help (perhaps other co-founders) to make up for an overall lack of talent in non-technical areas.
If you're willing to take a 70/30 split and let the other person simply "provide the idea" then I have about a dozen million dollar ideas I'd like you to take a look at. I'll be expecting my 70% in about 6 months, jsut call me when you're ready to make the first deposit.
As for the second part of your question.
What negotiation outcome should I aim for in this position as a technical co-founder? Is it wise to accept the offer, even if it is 50%-50%?
Are you starting a business, or just "making an app"? Lets assume you're starting a business. Considering you're going to need to bring in Talent (non technical talent), and at some point you'll need money, you need to be thinking in terms of how much more equity am I going to have to give up in order to get where we need to be. Lets say you start at 50/50, if you have to bring someone in to make up for deficiencies in non-technical talent that's another maybe 25% dilution, then when you need for sales, marketing, hardware, etc. you're looking at another 20% - 40% dilution, depending on the investment. Unless co-founder #3 is very strong and willing to be the CEO the VC is going to insist you hire a CEO, that's 10% on top of a standard 10 - 12% option pool for future non CEO employees. By the time you have hopefully gathered enough of the essentials you're going to find yourself with a 20 - 25% stake.
Personally, if it's successful, 20% is good enough for me. But you're taking a risk here as well. Chances of success are not 100%. You need to have a solid business model and understand the business side as well to determine if you're actually working on something that is viable. What's the sales strategy? Do you have to hire a support team? What do you have to do to be profitable? Does your profit margin rely on the founders not taking a dime in salary in year 1?
As the technical co-founder you should have the chops to crunch the numbers and figure some of this out if you can make reasonable assumptions.