I noticed that a sectional chart background is available on map images but it would also be handy to overlay traffic over low and hi altitude enroute charts.
We think so too! We were going to do it right after the VFR charts, but the way the FAA publishes them makes the more difficult and it got shelved for a bit. But it’s certainly possible (other sites have them online) and on our todo list.
+1 especially the High Altitude charts as a lot of the planes I track are the Boeing Test Flights
On overlay of actual ATC sector charts would be great too. These are the charts that show each individual sector, the same thing that Flight Explorer has.