Personally I won't be happy until I can customize my divisions from platoon level on up, including support units, and I expect to be able to assign not only divisional leaders but regiment, battalion, company, and platoon, and probably XOs as well since verisimilitude demands a higher casualty rate among low-tier officers and they should have replacements ready. It's probably best to just have each nation's entire officer corps from ensigns on up statted out and assignable. Cmon Paradox, this is a hardcore strategy game for hardcore elite computer strategist generals.
EDIT: We'll need squads customization to build each platoon, come to think of it. How else will we model historical FACT such as veteran GIs 'losing' their HMGs and mortars and requesting more, thus doubling up, or the increase in firepower to a division because Cpl. Täärni of Finland has captured a few new mags for his looted PPSh? For that matter, will different styles of magazine, such as drum vs. stick, be represented? IMHO militia should get special penalties in stick mag-using nations since they may grip their weapon by the magazine rather than the appropriate stock or barrel, reducing the effectiveness of their fire.