They should allow old and current seasonal pvp rewards to be obtainable by top 100/30/10 vouchers from any season. They could have Solo vouchers and Light Party vouchers and keep the rewards separate to each type of voucher. One reward for one voucher.
So like if in season 4 you get top 100 solo or top 30 light party or top 10 in either you can get any one seasonal reward from any season but only one per voucher from the current season or older.
Basically a list of rewards that keep getting added too each season and you just use the voucher from top 100/30/10 for whichever one you want in case you don't like the reward for that season and allows people to get what they want from older seasons. Not including achievements, titles, or trophies as they would stay as they are for placement.
Also that would keep people pvping for seasons that they wouldn't have because of rewards they don't like. Better for everyone.