If you want to go the custom engine route, you're gonna have to find an engine with a wrapper/bindings for DBPro, or you're gonna have to instead, use a medium such as Dark GDK, or GDK.Net to retain your easy to use DBPro stuff inside another language like C++ or a .Net language (C++/C#/VisualBasic) that has support for using that particular engine.
Pretty sure TGC sells a rather expensive (but still cheap I'm sure) engine called Leadwerks or something like that, which I think you can use with DBPro ?
The only commercial game series I know of that uses DarkBasic (or as the programmer puts it, a mix of Dark Basic and C++) is the Evochron Series.
There are other games within the same universe that are much older, such as StarWraith, etc.. But I think they've all been done on DBPro or DBPro/C++ mixtures.
Evochron in particular is a very impressive game for what it does, and for its file size. It has a small but dedicated customer base as well.
http://www.starwraith.com/main/go.htm
The newest game in the franchies is Evochron: Mercenary, which I am ASSuming is also built with DBPro/C++
http://www.starwraith.com/evochronmercenary/index.htm