The old building on site is being dismantled. Pictures
over here if you're curious.
This photomontage shows what the skyline should look like by 2025, as seen from the Montparnasse tower.
LA DEĢFENSE 2025 by
Vincent Montcuit, sur Flickr
This new tower appears stuck to the right of the Eiffel on the picture, while the other new one at the left end of the cluster is Hekla, currently under construction.
I think they add density to it, making it even more compact, but they're not tall enough to make any real difference in height.
Supertall needed here.