I wouldn't care to improve the system. I'd care to change it quite a bit.
imagine
-item blocks are now item shops
-each zone of blocks represents a different shop containing different inventory giving players a chance to access all of the same inventory
-different tracks will feature unique item block zones = unique stores = unique item inventories!
-the game could feature over 100 unique items
-a lot of items are best used against the track's obstacles rather than other players
-items cost coins
-coins are also spent to boost a kart's stats
-items can be stored in a rented item box (inspired by SMB3's Item Box) for later use in different races, but coins will continuously be withdrawn from your S account as long as an item is kept in the item box.
-coin bleeding rates from storing items vary from item to item, and the overall rate is higher the more items you're storing at once!
-because items can be stored, players can potentially use items on tracks that did not feature stores containing those items. In fact, the way all tracks would be designed, players will want to do this quite often (especially for access to special routes that are faster but significantly more challenging)! Reminder. Players bleed coins from their S accounts as long as they are storing items!
-cost scaling says race position and S account balance affects prices of items
-about C vs S accounts
-the C account is for coins to be spent during a race
-the S account is for the total amount of coins you have earned. Most of it comes from completing tough single player challenges that are distributed weekly by Nintendo (like Ubisoft's RL Challenges), but you can also build your S account by performing exceptionally well in online races (so much more to it than simply finishing 1st)
-when your S account balance crosses certain lines, you gain access to more of the ABSURD amount of customization options, which have no affect on gameplay (all cosmetic)
-of course, by losing and over spending for the race, you can cross back over the line, losing your access to the spoils... Oh well, time to do some more Weekly Challenges!
-there is a limit for C that changes from course to course, and all players have that limit
-C is ALWAYS built prior to a race by transferring funds from S
-when a race is completed, calculations are made and coins are deposited back into S
Of course, I've not thought this through thoroughly to cover balancing issues, but the point is I'm ready for a new item system. lol!