Combining biology and your existing skills might be a great place to start, as Al says.
Perhaps you could think about combining renewables with biodiversity? One of the big things in the very near future *should* be the UK catching up with continental Europe and integrating PV and solar thermal with biodiverse ('green' and 'brown')roofs. There is currently a large gap between what developers will countenance and what 1) the Local Planning Authority may recommend through condition(s) and 2) what we should be doing to enhance the remnants of the natural environment.
Someone with an understanding of both may well enhance either side of the planning divide (consultancy or LPA). The key thing which (in my experience almost never happens!) needs to occur is that biodiversity enhancements and (here in London) the 20% savings in CO2 emissions needs to be considered and evaluated at the very start of the design process. There is still a very entrenched position that there is an either / or situation; you can have a biodiverse roof OR photovoltaic / solar thermal panels. What I (and obviously, loads of other people!) am trying to do is promote the continental research showing that PV efficacy is increased through the addition of a biodiverse roof (through the creation of a more stable microclimate) and ensuring we get the best of both (biodiversity enhancements and more efficient renewables).
Personally, if I dealt with contractors / consultants that had more of a rounded knowledge on both renewables and biodiversity, I think we'd have a great many more and better developments, instead of being 20 years or more behind the best works of the Germans and Swiss. That isn't to say that we don't have some great biodiverse roofs in London (and Sheffield and loads of other places) but as a nation and a planning framework, we are SO inconsistent and SO adverse to 'trying something new'...