Out of curiosity, exactly what are you wanting to connect to the Teeny? Generally, the answer is the Teensy is underpowered for the job (not enough memory, too slow of a clock speed to pump out a full screen of data 30-60 times a second).
There are small monitors (1-3") out there that are I2C, SPI, or with direct pin connections that the Teensy can drive, but if you wanted a full size monitor, you might be out of luck. It may be better to go with something like a Raspberry Pi or Beagle Bone Black that have the specialized graphics processing unit, faster clock speed, and more memory to drive hdmi displays (R-pi can also do composite video, neither can do VGA). However, for VGA, you likely would need a hardware translator from HDMI/composite video.
For status and the like, I've used a 0.96" dig-ole OLED display (color 96x64 OLED:
http://www.digole.com/index.php?productID=882, mono 128x64 OLED:
http://www.digole.com/index.php?productID=528). They also make 1.3" OLED and 2.7" LCD modules that use the same programming interfaces.
Adafruit does sell a RA8875 board (
https://www.adafruit.com/products/1590) that you can connect to a 5" 800x480 screen (
https://www.adafruit.com/products/1596). They also sell a gamedunio 2 with a 4.3" screen (480x272):
https://www.adafruit.com/products/1654