There sure could be some unknowns out there, in isolated and inaccessible places seldom visited by plant people. Pinus maximartinezii was "discovered" in Mexico by botanist Jerzy Rzedowski in the 1960s when he saw huge pine nuts being sold in a market stall. The locals had always known about the pine, but no plant scientist arrived on the scene until then. Any other "new" species is likely to be known to local folks unless it is very similar to a related species and not differentiated except to a plant scientist. That's what happened when Pinus longaeva was carved out of P. aristata, though in this case foresters who should have known better had overlooked the differences for many years. Also, as molecular genetics provides excuses for delineating new species, those will be delineated.

....and what about extinct unknowns in the fossil record?
Isn't there likely to be a good (greater?) number of those too?

There used to be alot more species, every time there is an ice age a huge chunk of stuff is erased. So, there could certainly be more fossil discoveries.

China and Indochina area certainly are a HUGE area that has had a mild climate over time and has been difficult for science to "discover", at least in the past... how many unique trees have survived the glaciers from around here? Indonesia also has huge tropical areas "undiscovered", thou I don't know about the prospect for odd conifers.

You would think that Austrialia and the southern islands would have been scoured by scientists by now looking for relic oddities that have survived humanity.

I'd say the best chances are for something new would be a new species of Podocarpus somewhere in the cloud forests of the northern Andes, but that would hardly make news headlines.

For something that would make headlines, my choice would be a new species of Tsuga in the Sierra Madre del Sur in Guerrero or Oaxaca in southern Mexico.

Resin

Is there a fossil record of Tsuga in Mexico? The
extinct Metasequoia occidentalis from North America
could have found a niche in NE Mexico.

"Is there a fossil record of Tsuga in Mexico?"

None that I know of, but Mexico hasn't been very extensively searched for plant fossils.

Resin