Although I am no fungi expert and do not know exactly what type of fungal disease attacked your ajuga, if this was my garden I would improve but not remove the soil. Adding some sandy grit to improve drainage and cultivating the soil a bit to loosen it up and allow for air to circulate better in the root zone should help to prevent the fungus among us that thrive in damp, dark areas with poor air circulation. I would plant several different ground covers  this year and see what grows best before  making a commitment with a large number of plants. If the area is shaded consider pachysandra, vinca, saxifrage, Japanese Forest grass, cranesbill geranium or a mix of these. Add some stepping stones if you need to walk in the area and mulch it all with some wood chips or bark to seal in any fungal spores that may be left in the soil. I think you will find your disease problems are over but you will still need to hand weed the groundcovers until they fill in and keep ambitious growth of any groundcover under control. Keep growing, Marianne Binetti