It'd be extremely useful if Fiddler could have the ability to do filtering non-destructively, where filtering doesn't drop data/entries/lines altogether, but rather, merely hiding them from display.
This enables the ability for you to do multiple levels/layers/slices of filtering, as there's very often a need for doing on any given capture session. Currently, however, when you filter on something, the capture data gets dropped from the data/result set, lost altogether.
Process Monitor by Microsoft/Sysinternals has this ability, and it's extremely useful, allowing you to not only do layers of filtering, but also allowing the ability to traverse back up the "stack" 1..n filter layers, and if/when needed, able to un-filter all the way back up to baseline of all capture data shown (and without having to re-load a session save).
Procmon also has the ability to "Drop filtered events", which when enabled does destructive filtering, dropping any non-filter-matching packets from that point forward:
This would also be handy to have, but not crucial; much more beneficial/important is the ability to filter non-destructively.