Oh, I'm going to start work on my additions now, and splice them in when my turn arrives. I mean, why wait if we are each (apparently) going to code everything new for our additions?
Well, the main reason to wait is that you can use the previous players' materials, interactions, items and templates, so it gets easier and easier to add stuff (it's supposed to be a collaborative effort to build stuff from scratch, technically, not that every player does it all over every time). This is a little useful because it's surprisingly painful to make EVERYTHING even for a few races, as I've learned.
However, if you are going to want to control everything and build it all from scratch, say make your own dragon bodies, layers, equipment, attacks, materials, and metals, then there's no reason to wait at all. When the turn comes you'll probably want/need to fine-tune it to work and be sort of balanced with other races existing at the time, and maybe use their items too. The race balance is likely going to be sort of different from norm, for example my races have thicker tissues... at least had at some point... but also several hundred times higher velocity modifiers on their attacks so their internal balance is supposed to be similar, but I don't know how they work vs. vanilla. Likewise entities will have different ethics you need to relate or not relate to. If you have environment_spec minerals you may want to make them more widespread by adding them to existing stones. And so on.
Or you could start preparing stuff like interactions and bodies now, and simply migrate them to already defined materials when the time comes... something in between.
Damn that idea of mechanical dwarves had me excited by the way. You could have different castes with different augments. They could have actual axe blades hidden in their/instead of beards. They could release steam in combat. But you guys are more imaginative than me so it'll probably eventually be even better.