Does this mean I can only teleport TO a plant thats 10 feet away?

Or does it mean any plant within 10 feet of me becomes a door to another plant I've touched in the future?

"This spell creates a magical link between a Large or larger inanimate plant within range and another plant, at any distance, on the same plane of existence."

The casting requirement is 10 feet, which is indicated to be the "Large or larger inanimate plant". Then, you pick another plant, at any distance, on the same plane of existence, to create a link to.

So basically you second interpretation.

As long as it's still plant-shaped and connected to the soil, would a dead tree still work?  It doesn't specify that the plant has to be alive, but I imagine that a tree that's been cut up and turned into a house wouldn't pass muster.

I would say that a dead tree no longer works.

Maybe the spell draws upon the natural power of the tree and a dead tree has no more of that power.

does this allow you to take others like in dimension door or just yourself

Any creature can step through and it lasts a round so as long as they can move and want to go through they can. You could probably carry a creature through or maybe throw something through, at the DM’s discretion.

This is a cool spell, but also, not the most creative name

"You must have seen or touched the destination plant at least once before."

Does 'seen' still count if the plant has been seen via beast bond, scrying or clairvoyance? 

suppose so as technically youve still seen them

What if the plant were re-planted? Would I come out somewhere unexpected or would the spell just not work?

this is fast travel

That's a good point...

As a DM, that's something I'd do to a story-significant plant.

It has to have been a plant you have already seen or touched.

You're right 

How about Plantsport.

So is there an official ruling from Jeremy Crawford or someone about how many creatures can enter the Transport via Plants?

I'm also looking for it... but doing some raw math using a map here, safely we could pass at least 50 people moving 60ft on a coordinated move, in 6 seconds, considering they have 30ft movement (moving 60ft due to move + dash)... If I spent more time coordinating the move, I could come up with more people... if you add people flying as well, you can increase the number drastically because people would not have to step on the ground after going through.

Also, 50 people considering that even though the tree would be at least 10ft large, the space to go through would be only 5ft. 

Quote from BirdMonk >>

I would say that a dead tree no longer works.

Maybe the spell draws upon the natural power of the tree and a dead tree has no more of that power.

 I would say correct, as I see it, "plant" would imply alive as a dead tree is just carbon

Or “Transplant”

Quote from JustusandJustice >>

Or “Transplant”

 Too bad there's no way to upvote this!

I wonder if someone can be grappled and made to step through with the grappler, or pushed through, or flown through.🤔 If so battlefield removal for the win