the thing is you have to make it do a focused task. Not like: be more tanky, or do more damage. It has to do something specific such as: "An adc's rsponse to an AP burst mage" The problem with riot is they make everything too general.
The idea here is you have to choose the summoner spell, AND buy the item (most preferably the whole item) to get the upgrade.
Maybe something like Heal: Now heals for percentage of health and gives AD/AP equal to 10% of amount healed for 5 seconds.
OR Heal: Now casts globally on all allies
Maybe have different items be strong in different stages of the game, cheap upgrade to barrier to make the range global and target cast instead of self cast.
Also perhaps you dont even need to upgrade ur summoner spell, it depends on when and where. You CAN sell items you know in response to enemy builds you can sometimes use an upgrade and sometimes not. No you wouldnt need another item slot that defeats the point, the upgrade needs to be niche (which i might add should be the case for any item)