Try sweet woodruff with spring bulbs. It does especially well under deciduous trees which leaf out and provide some shade during the summer.

White forget-me-not. If your timing's right - White Nancy Lamium.

For me, Jetfire has usually gone over before the white Epimedium starts but it could be a possible.

If it doesn't have to be flowers - the early leaves of Brunnera Jack Frost can be good, or Majeste Pulmonaria.

Trouble is - Jetfire is such a successful multiplier that the clumps need fairly frequent management so a long-term perennial is not always a smart move (telling myself, and probably not listening...lol) Another possible is white primroses with a strong yellow centre, or a soft yellow primrose. They make useful clumps and it's easier to manage the bulb groups than if the groundcover is total.