I have an order for a wedding cake. It will be 3 tiers, heart shaped. The order is fondant covered and border made with fondant heart shapes. She wants tiers betweeen each layer,not stacked. My question is will it look too plain with nothing else on the cake?

Will each tier be elevated on pillars? If so there really should be something in the center of each cake.....if nothing else than a (standing up) heart.
We are here to do what the bride/customer wants unless it is impossible. If they want a plain cake........so be it
I had to do a chocolate cake - 3 stacked layers with *nothing* - no border, no nothing!

What type of standing heart, Kakeladi?

Just some dried fondant cut-out. Put a toothpic in the point so it will stand up. A grouping of the would be nice.
I'll post that choco cake in my pix in a few minutes for you