The only successful people I know that run 'side' businesses work at them 110% putting in crazy hours for some good returns. One friend runs a company that gives out branded chilled desserts on hot days with advertising on. Maybe this will turn into his retirement money, but for now it gives him a good lump of money, but takes serious hours and head space alongside other commitments. And there is no guarantee that it will still make him cash next year.
I know specialising can feel like selling out, but that it where you look to specialise is something you are passionate about and care for.
Savings, maybe property in the right area and stocks it you get lucky is the way to go likely, all backed up with a solid career.
But then again, I'm a wage slave so what do I know! Go for it and let us know how you get on! Id imagine it would make a brilliant blog!