I wonder what was unhelpful in my post?

For the described problem, starting with Doom2 would be my choice.
If it's not enough with just new resources, you could even add functionality too it, since source is available.

Here's a list of WAD editors: http://doom.wikia.com/wiki/Choosing_a_WAD_editor
Here's even more tools: http://www.doomworld.com/classicdoom/utils/editors.php   and you should also be able to find information on how to do it on that site.

You could make a completely new stand-alone game with these tools, as long as the doom 2 feature set is enough.

I wonder what was unhelpful in my post?

 

For the described problem, starting with Doom2 would be my choice.

If it's not enough with just new resources, you could even add functionality too it, since source is available.

 

Here's a list of WAD editors: http://doom.wikia.com/wiki/Choosing_a_WAD_editor

Here's even more tools: http://www.doomworld.com/classicdoom/utils/editors.php   and you should also be able to find information on how to do it on that site.