Items that add an ability to your summoner would be quite powerful. Like +1 Speed boost.

I think have items that use an ability effect on a selected enemy creature for one turn, like stun, blind or snare would be fun to use. Of course, having access to using abilities on your own monsters would also be really fun, and easy to set up interesting combos or synergies.

Your post inspired me to look at all 64 abilities in Splinterlands and imagine they were all ability cards that I could use and assign before battle and it can get pretty wild pretty quickly.

I also considered items that fill empty slots like: SCARECROW: Fill an empty slot on the battlefield with a monster with 0/1
FORCEFIELD: Fill an empty slot with a 0/1 with Reflect.
BOOBYTRAP: Fill an empty slot with a 0/1 with Thorns.
Etc.

I appreciate the post. It helped me start thinking more critically about what these items might be like.