The soil in the area of the former pine is in the acidic range due to years of infestation with pine needles and is not a suitable location for certain plants. If you want to plant other ornamental shrubs, the soil must either be replaced or the planting hole should be generously improved with compost or good garden soil beforehand.
However, there are enough plants that prefer an acidic site and you could plant those right away: Rhododendron beds, for example, can be planted well on the “coniferous soil” (if, for example, a hedge or other woody plant provides shade). If you put a thick layer of bog bedding substrate on the presumably rooted soil (about 20-30 cm), these plants would certainly thrive as well: lilies, heather, ferns, certain cranesbills, and hydrangeas.
If the pine root remains in the ground: In many gardens you can see flower pots or birdhouses (like lighthouses and windmills 😉 on the sawed off trunks, is ultimately a matter of taste… We can only advise you to saw off the trunk as close to the ground as possible. Perhaps it can be concealed with ivy or with an overhanging, lush grass? (At some point over the years, the trunk and root will be decomposed by soil dwellers and fungi, but it will take a little longer. There are also “accelerators” available commercially called “Root Ex” or “Root Shark.”)
If you want to completely redesign the garden and think of completely different plants that need light, sun and a humus-rich and neutral pH, you will have to dispose of the pine trunk including roots. In addition, soil replacement will also be necessary.