Beyond Radiant - the keys to true level design.
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Radiant's entity system can be used to do a lot more than just create environments, as advances in lighting being good enough to use Radiant for that too, with simple exploits to light entities with custom keys and values. It's even possible to make use of DarkRadiant's pathing system to export interpolated vectors to use for things like camera trajectories, and light-meshing another example of Radiant being used for things processed only in another software altogether.
Traditionally Radiant always functioned like this, to handle as much of everything possible, but had been relegated to just creating environmental geometry, and no longer used for full-fledged level design, with newer engine editors and Radiant's idTech legacy not having much to do with each other. While it's only a tiny aspect of what Radiant is used for, things like to decide where pickups and enemies are located, it's still important for a mapper to not just feel restricted to only being an environment artist but also a true level designer in full.
Beyond lights, other "non-Radiant" features are possible to create/insert, such as basic particle systems and other effects. Some are brought into Radiant for placement as pre-existing assets, others can be generated from a cubic brush entity with a range of custom values which then become volumes and parameters. Neither of these methods provide full flexibility, but definitely a case of something is better than nothing, and mappers are able to achieve a lot from within Radiant, at the cost of a lot of necessary imagination and viewport litter.
Due to advanced data management, the possibility for Radiant, Blender, the engine(s) and much else to more directly interface with one another, even including manipulating binary files if necessary, as is the case concerning high-level rendering with Mental Ray. Considering the opposing view, it's now possible to do things like capture an entire real-time scene frozen in time and exported to a model, such as to display a particle emitter result, and converting transform rotation matrices and centered origins with scales to linear bounds. This means that anything conceivable and generated, even deep within an engine or rendered viewport, can be displayed in Radiant, and a mapper gains a lot clearer insight into the final result, while at the same time having much more control over everything.
A combination of Radiant having multiple branches, being open-source, DarkRadiant having an editor console, support for Python scripts, all using human readable text-based data formats that we're already very familiar with and a fool-proof exporting process, all means this is only the beginning of potentially many more great things. All of this combined with advanced data management mean the future possibilities are truly endless for Radiant.
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