Bullet impacts on different surfaces (ice, snow, concrete, metal,glass mostly) alone is worth a separate pack to me. Also a good looking explosions pack; flash, grenade, rocket, emp, mine, C4 would be certainly useful. I think both of them belong to a separate set because if you need one you’ll likely need the other ones.
Personally I’m not interested in rain or snow and stuff like that unless they are usable. For example if you have a rain particle emitter supposed to be attached to the player pawn (so you don’t have to emit in the entire map) you need to have a good solution to stop the rain when the player goes underground. Furthermore in the case of rain it would be important to provide a shader that simulates waterdrops and another that simulates dripping water on walls, this way it would be a ‘turnkey’ solution everyone could learn from and adapt to their own needs.
This said good side blowing wind particles like sand dust, snow dust and stuff like can be handy in many scenes.
Personally for me buying a particle pack would be half for the particle quality and half for the technical aspect behind them. I would certainly be more excited to buy your next good looking particle set if I know I can expect to see some voodoo/smart ideas in them.