20 Liter displacement?
16+ cylinders?
Extreme pressure supercharging/turbocharging?
15,000+ rpm?
Maybe some other developing or still unknown technology?

As insane as 1500+ hp sounds somebody will make it.
As amazing as it is, the Veyron will not reign forever. That's for sure.
When, whom, how and for how much are the questions.

Feel free to guess at it.