This would be for after the extended individual flight history is implemented;
It would be nice to be able to specify how far back one wants to search for an IFR plan between two cities, especially if flights between the two airports are few and far between. That way, even if there are no flights in the default time frame, there are better odds that at least one flight some time back went between the two airports.
What time ranges are you proposing? Also, have you tried searching nearby airports or are those results unacceptable?
For the two airports I was trying, KEUG and KPDT, there aren’t exactly any close airports to work against 
I suppose how far back would be up to you guys, but definitely more than the current setup. Perhaps a month or two?