One of the features in many of the areas in Stalkersoup are railroads/railways - rails. A DLC could be based on a rail network. Much would be in tunnels but using a portal system, a small single unit railcar could go through transitions.
A rail modeller might be able to organise this - a program like Auran's Trainz might give some ideas, as it uses a very similar to Lua construction of landscape. Since cars can be driven anywhere in the Game, it follows so too could a railcar from the type where a lever is manually lifted up and down to a diesel motorised unit fixed to a rail route. The tunnels in the Red Forest show a narrow guage network compared to the regular gauge ones eg in Pripyat.
There are several examples in the Metro series.
Storylines could be about such as illegal mining of uranium - Ukraine has this mineral, escape, to groups infiltrating areas.
Anyhow, anyone currently working up a storyline may find this suggestion useful I hope.