The reason I decided to suggest this is because of lure. I can't tell you how many times I've directed /tpahere to the wrong person, and having to worry about being lured. It would be a simple problem solver to add the command /tpcancel. Here is a senario in which it can help(all usernames are made up or used just as an example if they really exist):

Without /tpcancel :

Jollyworld70 is a vip, in wild looking to mine with her friend. Her friend's name is Doggies20. When she sends the tpa to Doggies20 this is what she types in /tpahere Doggi. /tpahere Doggi is directed towards Doggiesforever instead of Doggies20. Jollyworld70 suffers from her mistake when later her and Doggies20 are mining. Doggiesforever accepts the tpa in all diamond armor and with a diamond sword. Jollyworld70 and Doggies20 are both unprepared for this, having only stone tools on them and their mined goods. Doggiesforever gets banned for luring but the damage is already done. Jollyworld70 and Doggies20 have both lost all their mined items including diamonds. They are enraged and wait a week before playing the server meepcraft again.

With /tpcancel :
Jollyworld70 is a vip, who wants to mine with her friend Doggies20, so she's in wild. She types in the command /tpahere Doggi . /tpahere Doggi is directed towards Doggiesforever by accident. Realizing her mistake, she takes precautions and does /tpcancel. Her and Doggies20 have a safe mining trip and now have 20 diamonds, 128 iron, etc. They decide to play again tomorrow.

The Difference is that both Doggies20 and Jollyworld70 play again the next day, increasing the player amounts on meepcraft, also a mod didn't need to be called to help and Doggiesforever didn't commit the crime. They got to keep their stuff and didn't rage quit. To those who say it was Jollyworld70's fault that her items got taken away and she got what was deserved, think about how you would feel in this situation, or you being Doggies20 and it wasn't your fault at all.