Here are some of my thoughts:
Primarily it needs to be really aero: very tight so that, bent over, there are no ripples in the fabric to catch air flow. It should be tight enough that it's uncomfortable to stand up with it zipped. (I find many skinsuits far too long in the torso, so they bunch up when bent over on the bike.)
Personally I hate collars on speedsuits/skinsuits. They feel like breathing is restricted, or that I'm very subtly choking. So I like speedsuits to have the low collarbone look. Castelli bodypaint is a good example.
A thought: for track and TTs, a rear zipper is better than a front zipper. Have a little pull tab on it to help with zipping/unzipping oneself.
Leg compression would be nice, but not if it makes the suit hot/not breathable.
High heat areas need to be particularly breathable: armpits, lower back. I like thinner material all over. I find Voler's long sleeve speedsuit to be comfortable even in 100F temperatures. It feels cool from sweat evaporation through the thin fabric.
Some suits are using a thumb loop on the arms to help keep the sleeves tight and in place.
And if some consideration can be given to where numbers are likely to be pinned on a speedsuit: a single number middle of lower back for TTs, and often one on the left lower back for track (though two for higher level events).