That's a metal cap, not plastic, but yes I'm interested in what kinds of modifications I could do to it in order to discharge into it.
I live on the California/Mexico border in the desert and we don't have any water softener specific regulations like you'd find in the coastal and populated cities. For better or for worse, imagine I'm in 20th century Mexico as far as codes/regulations/compliance. That's not to say I ignore the rules, far from it, just that I seem to be very rare in my region resulting in 1) not very many licensed contractors in the area, 2) not very many permitted builds in the area, and 3) apathetic inspectors who sometimes need to be reminded of the code they're enforcing. Completely unrelated but by way of example of the lack of regulation where I'm at in comparison to the rest of the state--we don't require vehicle smog checks.
In any case, my question is if I can modify that pipe (and by modify I am thinking screwing something into the threads when I remove the cap and not breaking concrete to get down into the ground and actually change it out) to vent from my roof with a horizontal trap for my discharge line, can I simply raise the pipe just enough to place that trap like say 4-10 inches? Or do I have to run it up to the roof and leave it uncapped so it can vent in order for the trap to work?
The more general question regarding the space is that the door is near useless other than to remove/replace the water heater. I could easily leave the door intact and locked for the next time I replace the water heater or I could close it off and drag the beast under the opening and use the actual side door. I don't know why someone put two doors there!