There's perhaps a number of potential industry sectors you could go into and use your dual US/UK citizenship to good use. That however will also depend on what your interests or experience and skill set covers both from the past and what you might want to get into.
There's perhaps more but those are just quick ones off the top of my head. You'll need to find an opportunity or ideally a niche where there's demand for expertise/knowledge/experience of both countries and particular industries where you feel you could make money from.
I think you'll find it harder than it initially appears, but then you might already have a 'plan' or 'focus' already !
There's not much traffic/posts on this forum for the small business/entreprenuer but plenty of other sites and resources you could look to get some ideas and knowledge from.
What would you ideally like to be involved in for your idea do you think ?!!
For me, my educational background is Engineering, went into Financial IT as a career for 12+ years, then into helping to run a family business that started with 3 of us and rocketed in terms of expansion within a short space of time, went into Specific/Corporate Consulting for a year or so and now have indeed found a niche market for which I've made the change for a niche manufacturing/export opportunity I spotted some years back.
Cheers, DtM! West London & Slough UK!