It's not really my mod, I've just been contributing a lot to it because IMO the Nether should be epic!
If you're judging the darkness by screenshots I've posted, they have some gamma issue making them appear much much darker on other devices. However, until the nether fills with lava it can be rather dark (a challenge?), it'll have a lighter appearance once the climate_api PR is merged due to distance fog not fading everything black, but that's probably not what you meant.
The nether doesn't seem like a place for conventional plants (sort of) though I did always want to start with some sort of Nether weed, perhaps found growing on dungeon walls, or maybe around fumaroles. As for ruins, the dungeons are getting better (and are all ruined to some extent), but real nether structures would be great. I'd love it to have treasures too but what can you add that's valuable in MT? Things with real value are useful, which requires coordination between mods rather than every mod spamming diamond/mese or proclaiming its own blocks a treasure. Perhaps the fumaroles count as a Nether material for building - they can be used in overworld cottages to have smoke rising from your chimney. If PR10141
gets merged then silk touch tools become possible in Minetest, and the Lava Crust block would become able to be mined and used. I agree about new materials for building, but ideally they introduce more than a different texture.
It's not my mod so it's not my call, but a game design principle I'm trying to follow is that new blocks (and plants) are best introduced sparingly and by considering extra uses and things they could bring to the game beyond being essentially a different color of something, if it can't be dual-or-more purpose is it necessary? (often the answer is still yes, but it's good to ask the question).
e.g. a nether weed would ideally enable some sort of recipe. More content is needed, by more people, and we should be bouncing these ideas off each other - like the ones you and ywwv posted.
The nether already has a new lava, because default:lava looks like a 16x16 patchwork when used in oceans, so perhaps it could be a golden lava. What would golden lava do? A lava that doesn't spread/flood as much might be better for the nether, perhaps all these could be rolled into one new nether lava block. Currently the nether lava drops as default lava in a bucket, should that change?
Nether dungeon from upcoming PR, lightened to counteract my gamma issue.