You can approach it a couple ways. Here is one...
FIRST: calculate the bearing and distance to the two 'outer' points from the anchor point. Here are a couple helper functions:https://github.com/gregallensworth/Leaflet/blob/master/LatLng_Bearings.js
SECOND: adjust those bearings however you want...keep the original distances.
THIRD: Using the new bearings and the distances, calculate the new LatLngs using the accepted answer here: How to calculate the latlng of a point a certain distance away from another? (This uses google maps instead of leaflet, but its easy to port to leaflet)
Let me know if you have any problems implementing...
