jfer - The best advice I've seen is just keep saving. 401k to match, max a Roth, and then if possible then go back and max the 401k. That last one is pretty dependent on a number of things.
Some of those things are answered by taking the following steps.
1. Figure out what FI will be like for you. What will your life be like in the 10-15 years when you may choose to leave the workforce.
That question will lead you to #2. How much will that lifestyle cost?
Once you have a number expressed in both annual costs and a lump sum (costs x 25 [or 33 for a more conservative take]) you can start thinking about how that will be funded.
That funding will generally take the form of investment returns, rental income, or business income. What you seem to be leaning towards is the investment side. And you seem to be concerned with how to access money in your retirement accounts prior to age 59.5.
This can be done in a few ways. Look up 72(t) for rules on early withdrawals, and you can do a search for Roth pipeline (do the search either here or on the web). There is no guarantee that either of these options will be there in the future. But then again the future is never a guarantee for anything. :)
So that remaining money after you've gotten your match and the Roth is fully funded can either go back to your 401k for fully funding, into a standard investment vehicle, or savings towards some other strategy like building businesses, buying businesses, buying properties for rent...etc.
