The Shuttle will be replaced by two different platforms; an SRM based one to carry people and a Heavy Launch Vehicle (liquid core w/ solid strap-ons), to carry up to 100 tons of cargo,ie Saturn 5 class.
Both these systems will have the payload at the top of the tanks and propulsion system, to minimize damage from ice and other solid debris.
They will also include a much more advanced launch abort system then has been imployed in the past.
The Shuttle is a marvelous machine, but it was designed in the 1970's....