I think it would be cool to have a choice, if possible. Its less likely to make 1 item that everyone wants. Instead of suggesting specific items as requested, I am simply asking for a choice.
1) A choice between unique (even already existing) color crystals with expertise. (A Pair of crystals)
2) A choice between various pairs of pistols (or whatever weapon your class uses). These can be existing ones with particle effects added. (Revolver style, ray gun style, modern pistol, small gun, big gun, etc)
3) A choice between various helmets with particle effects on the eyes, or where ever looks cool. These can be existing helms with effects added. (Only a choice of helms that fit your armor type theme possibly)
It would require a bit more work, in terms of more designs, but it's not a halfbad idea.
I'm imagining a "season vendor" with a few basic rewards and then the cool ones from current and previous seasons. So instead of earning the rewards directly we're getting tokens sent to us, based on our tier. Everyone who gets tokens can choose to spend them on the basic rewards instead of the new seasonal ones or simply save them until the next season. The cool stuff such as season mounts and whatnot will also require the rating so a person won't be able to save the few comms they have from the lowest tier a couple of seasons and then buy the highest tier's mount. The pro with allowing people to buy old stuff is that new players will have something to aim for even before the new season rewards have been announced. The highest tier will get enough tokens every season to buy all the new seasonal rewards.
A good start would be to add 20 basic rewards + the season 1 rewards to the vendor and then add the season 2 rewards and perhaps a couple of basic rewards every season.