Any change that allows the use of a larger part of the TKD spectrum of techniques would be an improvement. Punches that actually score, both to the body and the head, just for one, very simple change.
TKD is not so much a ground fighting art, but allowing sweeps and throws with the chance to follow them down for a strike (and a score) is another option.
The inability to effectively use the hands while sparring is, in my opinion, the single biggest weaknes of the WTF ruleset.
The biggest weakness of the WTF rule set is it forces the practitioner who competes to perfect their individual training schedule in order to be the best at kicking, punching, stepping, dodging, clashing, covering, timing, distance, will power, strategic thinking, deception, etc.
This weakness forces practitioners to achieve their highest potential in order to even survive through the various levels of WTF competition, it's a life commitment. So many who try WTF competition realized they have to focus on that skill set with a Zen like commitment. At that point most realized they do not have that type of commitment to Taekwondo and drop out of competition.
The weakness is that it is very difficult to achieve the depth of skill, knowledge and experience to be the best at kicking, punching, stepping, dodging, clashing, covering, timing, distance, will power, strategic thinking, deception, etc.
It is very easy however to create excuses for not being the best at kicking, punching, stepping, dodging, clashing, covering, timing, distance, will power, strategic thinking, deception, etc.