A list of some possible new stuff I thought of, in no particular order:

Mist-Shrooms: large mushrooms that use the world shader to render clouds of mist on top of blocks around them. Spawn around spirit trees, blanketing the ground in fog, improving the ambiance.

Brew of Ghostform: allows temporary flight and no-clip, but you are unable to interact with the world, similar to spectator mode. Additionally, your view is obstructed while your head is in solid blocks. Protected blocks and energy barriers are solid to you.

Proper names/icons for herblore effects: currently, the betweenlands' custom effects are named after the brew that inflicts them, and have a bottle for an icon. They really should have unique names and icons like vanilla effects IMO. If this is already planned, please ignore this.

Sinkholes: deep holes that reach down to the pitstone caves. Can be found rarely in sludge plains.

Pebble bombs: barrels full of angry pebbles. Explodes instantly when powered with a redstone signal, making them great for landmines. Barrel starts empty, but can be filled with pebbles to increase explosion strength.

Drop snakes: blind, snake like creatures disguised as hanging vines. Upon contact, they wrap around you and start dealing damage. Come in surface and cave variants.

More herblore contraptions: I really like the repeller, there should be more devices like it. I hear theres a crystal ball in the works, but more devices would be great. Perhaps a furnace that runs off fire aspect, or lamps that confer effects to surrounding mobs, like a lamp that makes animals more willing to breed.

Cool suggestions, I like them!
But we have so many things already planned out, I don't think any of these will happen soon :P
I can however tell you that we've already got some really cool and strong/useful stuff planned for Herblore

Thats great to hear! I love the herblore system, its implementation is very clever, and makes you feel like your actually doing alchemy. I would love to help ease some pressure, but you guys are kinda out of my league code-wise lol.