Ah! The age old dilemma. How do you make a fluffy army that is also competetive?
I think the first thing you should do is go through the
SM dex (we're stick with Legio VII here so no variant lists). Traditionally, the fists are masters of defence, seige warfare and anti-armour/tank hunting but remember they are a codex chapter and will adapt to their circumstances - its not all
dev squads and ironclad dreads.
Pick a theme that you like and make a short list of units that would fit with it. Post those here and we'll see what can be made out of them. Otherwise, this thread will become a list of
SM Interwebz lists...