Hi,
FME already supports (b3dm) batched 3dtiles format, however does not supports i3dm (instanced 3dtiles). As you know, b3dm tiles are most used format to represent buildings, city objects in CesiumJS environment. I3dm is also another 3dtiles format, mostly suggested to representing 3d models sharing same geometries like trees, billboards, city lights and etc. FME have lots of essential and powerful transformers to create instance 3D models and calculate coordinates of these models (for exp. RandomPointGenerator and PointOnRaster ValueExtractor are very useful to distribute these models on to map). I think it will be a great feature to support i3dm 3dtiles with ability of combining multiple tilesets into one, like in this sample: https://github.com/AnalyticalGraphicsInc/3d-tiles-samples/tree/master/tilesets/TilesetWithTreeBillboards