So I have a back story, and the figs (Shelving half my Tanith, Cannablizing the rest, mixing in some new units) but I am looking at what and how to build the base list.
I am tempted to just use the book as is, and not bother with Doctrines, however it is early enough in my thinking to consider all sides of this coin! Without typing a novel: Heavy Industrialized High Gravity World, but not quite a Hive World or a Forge world, but almost there. 4 Continents, one which is very large and mostly unsettled. One which is small contains a few hives, and most of the worlds population and production centers, and two that are somewhere in-between. I have toyed with placing it near Cadia, or the Eye.
Doctrines I have looked at: Veterans, Grav-Shoots, Sharpshooters, Chem-inhalers. I would like to consider using Sanctioned Pyskers, Rough Riders, Snipers, Special Weapons, and Heavy Weapons.
I am dead-set on at least using my Catachan Snipers(Ratlings) and my Heavy Weapon teams, which would leave me with three Doctrines/choices. (Also My own World!

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For models I have, quite a few Cadians and Catachans. My Catachans used to be about 50% all Tanith, I have stripped them all down to grey models again, save for a few heavily modified and
GS dudes. The other 50% of my Catachans and my special purchased metal Tanith models will stay as my Tanith and Shelved for the time being.