Really been enjoying xcom recently, looking forward to giving a lot of the mods a go (like the look of MiB and the Alloy Interceptor) and might look into modding myself if it isn't too tricky. But here's some stuff I was thinking...
The Bullpup is a light upgrade of the standard rifle that is bought.
The Gauss weapons are inaccurate slightly more powerful laz weapons.
The plasma weapons are made for humans allowing for greater accuracy.
The alloy shield is a one handed item that increases front facing armour.
The sonic blasters are a thematic change for sectoids, they can't be used by humans and deal great dmg up close, but dmg that is weaker over distances, also they are very light weight.
Armoured Sectoid. Though they might prefer otherwise sectoids can and will wear armour when the situation dictates it must be so, soldiers can clad themselves in their alien alloys though their movement becomes quite costly.
The flamethrower is a fire vomiting device.
Standard Military Armour is means of added protection early game, however the weight can render it obsolete against most alien firearms.
MKII Power Armour: Enhances the trooper even more than the MK1, the Mk1 would receive a de-buff and become slower moving though perhaps with grater protection.