1. Something to determine whether FIDE Law 8.4 applies. (If a player has less than five minutes left on his clock at some stage in a period and does not have additional time of 30 seconds or more added with each move, then for the remainder of the period he is not obliged to keep his scoresheet up-to-date.)
2. Wireless connection to other devices it needs to communicate with, or at least wiring that is routed so it doesn't get in the way of the players (unlike the badly designed DGT clocks that have a cable coming out of the side of the clock and into a port on the board on the border of the a2/a3 squares).
3. Functionality updatable via flash ROM, or similar, so faults can be rectified and updates applied if FIDE Law changes make it desirable.