Plants do not get a lot of love in D&D in my opinion. There are only 13 plant creatures in the entire Monster Manual and only 4 of them are CR 1 or higher. The lack of love continues in the Dungeon Master’s Guide with only 2 types of Mold, and the poisons and disease section don’t portray any association with plants at all. This must be remidied!
Plants make amazing natural traps. You can reskin most traps to appear as vegetation with little effort. A poison needle trap could be an aggressive cactus, Fire-breathing statue could be a plant with explosive seed pods, and the rolling sphere trap could be ginormous coconuts. These are just the traps in the DMG. You could have magical plants expel spores that cause magical effects which opens up a world of other possibilities.
Another easy way to get bizarre plants in your game is by taking just about any creature in the Monster Manual and just calling it a plant. What was once a troll is now a regenerating cantankerous dogwood tree. Get weird with it. You can turn a Grell into a floating rosebush with long thorny roots or a Magmin into a flaming tumbleweed that explodes.
The creature could make a great guard since most plants can’t move. Want a Harry Potter romping willow? Make the Stone Giant a plant, reduce its speed to 0, and done.
Spores & Poisons
Plants are known for doing all kinds of crazy things to us in the real world. I can only imagine what they can do in a world filled with magic. It is totally within the realm of possibility to have a mushroom expel spores that dominate nearby creatures to kill each other to they can rot and give it nutrients, or have a simple berry bush protect its magical fruit by repelling predators with the Antipathy spell or with Otiluke’s Resilient Sphere.
You could also hypothetically allow some plants to be used in the construction of potions and poisons. Heck, you could have branches be magical staves, wands, or weapons or used in their construction.
I’m really just scratching the surface of the endless versatility that plants can provide to your game. I do hope I’ve at least planted a seed of inspiration for you to nurture.