The features will depend on what you will be forging and your goals. Will it be a sedentary shop smithy based forge and or one that you want to transport to other sites to forge with? Do you want a hand or foot operated air source or an electric blower? Will you just being doing blades or assorted smithing of various shaped steels? How large might the pieces of steel you want forge possible be? How much smoke and fume venting/exhaust needs will you have to address, in other words just forging outside or will you be working in a more enclosed space?
For my part, I enjoy a fairly good size and deep fire pot set toward one end of a large table with plenty of space for extra fuel and to lay steel or a tool or two on from time to time. Here's a shot of one...