What are your thoughts about a split between CS:S and CS:GO?
==> Common stuff stay in #include <cstrike>, while the differences go in #include <css> / #include <csgo>.
-) It will break some CS plugins (New includes to add, new weapon/item enumerations to use...). These plugins will certainly become CS:S / CS:GO only. +) You will be able to do whatever you want in the future (=> Remove completely WeaponID for CS:GO ; Create specific natives for CS:S / CS:GO...).
Valve will certainly continue to update CS:GO and add/modify/remove stuff. Issues like you explained above could happen more and more...
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About adding new natives, could you please provide the vtable of the classes CCSWeaponData / CEconItem (Remark: these natives will certainly be CS:GO only, right?)