In inclement weather, or dark evenings, people may simply assemble in each others' houses. From an environmental point of view it's doubtful whether a separate building is justified. Shared use of an existing building might be better than a purpose built place solely for devotion. Some of the elements always found in nature would be present at indoor meetings too. Pantheism is a religion of reverence for the Universe and nature, and every meeting should have some reminder of that as a focus for meditation.
Fire is already a symbol for some religions, but it is specially appropriate for pantheists.
It is important for us today as it was for Heraclitus, and for the same reasons. Fire represents the energy and radiance infusing all things. Fire's constant change embodies the metamorphosis and flux inherent in life and the universe at all levels. For more focussed attention, records of birdsong or whalesong or the sounds of an awakening forest might be helpful. Natural objects may be passed around, objets trouves , shells, crystals, pieces of bark or root, pebbles.