If you want to suggest a new feature or vote for one, you can use our New feature request website
Pebble Bike already support BLE (Bluetooth Smart, or Bluetooth 4) Heart Rate Monitor.
ANT+ may come later, but it is not our priority for the moment. Lots of phone manufacturers are moving from ANT+ to BLE. Sony is the major one (2012 Xperia phones were ANT+, 2013 only support BLE...).
Altitude accuracy is usually 3 times worse than horizontal accuracy.
So a GPS accuracy of 5 meters generally means an altitude accuracy of 15 meters.
If your phone has a pressure sensor (Galaxy S3, Galaxy S4, Galaxy Note, Galaxy Note 2, Galaxy Nexus S, Nexus 4, Nexus 5...), we use it to improve altitude accuracy.