In addition to the tree types, some kind of magma-resistance wood type would be amazing. Perhaps only the elves would have access to such an item? It'll certainly give them something to do! As well as being able to carve wooden armor, wooden weapons from the harder varieties of wood.
Spores are a fascinating extension of the multi-tile items. Perhaps a type of soporific, causing anyone who ventures too close to the fungi to fall asleep and, eventually, die. The presence of animal skeletons would probably tip off the observant adventurer!
Algae growths in standing pools of water (fouling it, perhaps) would be interesting. Also, pumping directly from a stream might occasionally or gradually cause the pump to become jammed, either due to seaweed / fronds gumming up the mechanism, or a dead fish (there's a carp stuck in the screw pump again!)
Soft sand! Treating sand as a type of fluid (with 6-7/7 sand filling the tile, rendering it walkable). Sand would 'flow' but only reluctantly. While digging into sand would be difficult (the sand rushing in to fill the hole as quickly as the dwarf can dig it out), a sand 'leak' would probably mean the loss of 5 tiles or so. Perhaps treating all sand as having no pressure, and sand only 'tumbles' from high to low?
Example : one tile hole in the ceiling
1 3456543
1 121 4567654
121 12321 etc. 5677765
1 121 4567654
1 3456543
When all adjacent tiles to the breakthrough fill up with sand, the hole is considered 'blocked'. In addition, sand only spreads out if the sand within one tile is more than 1 higher than an adjacent tile ( 31 > 22, but 21 would be stable).
Sorry, got side-tracked in mechanics land : but this would certainly make playing deserts fun! Add into that shifting dunes, and you'll get Urist Ronavlud Cancels Sleeping : Entombed in Sand in no time.
EDIT : Also, meteor strikes. Perhaps having them as an adjustable frequency within the init file. A huge hunk of valuable meteor matter slamming into your fortress? Great! It took out the Countess? Even better! However, these should be incredibly rare in the first place, otherwise it'd just get silly.