Doran's items are balanced. 5 of them is 2375 gold for 50 AD, 15% LS and 400 HP. While those stats are fantastic early game, that's all they are good for.
They're high cost-efficiency at the cost of being useless after the lane, pretty much. People keep bringing up the Prospector items to compare, but really there's a large difference. Prospector items are only available in Dominion, which is an early-game based map. Furthermore, Prospector's Blade/Ring are both basically twice as effective as Doran's Blade/Ring.
The only way to balance the item if you make it unique is to make it stronger since it can't stack. And lowering the stats would only justify leaving the stacking available (although it'd make using Doran's items pretty much worthless in the first place). Leave them be, Riot has balanced them out well.