Wedding cakes and miniature cakes are covered in marzipan or chocolate ganache and sugar paste, I find it helps keep the freshness of the cake and adds an extra taste dimension.
Combining the flavours of cakes in each tier is a great idea, why not serve the cake as a desert at the wedding reception. If some of your guest's have food intolerances you can introduce mini cakes or one of your tiers can cater for their needs.
Here are a few ideas but if there is something else you have in mind then just let me know,
there are no limits!