Here's an idea i had a while ago that nobody cared, i still think it's good though
Hold items that not every pokemon can use
Example: Blade: In battle: raise the damage done by slash moves (Cut, slash, night slash...) outside: of battle cut small trees.
Gloves: In battle: Raise power of fighting type moves Outside of battles: Rock smash
Dumbell: In battle: Double Evs gained Outside of battle: Strength
Saddle (Replaces bike) In battle: (no idea) Outside of battle: ride a pokémon (Who needs a bike when i can ride entei?)
You can always remove them before an important battle, and could introduce a lot more situations because of it, stuff like rock climb, or jumping to upper ledges. another alternative could be this: cut becomes a TM and holding a blade raises the power or crit rate of cut.