Enable the option for players to transition their existing characters to a seasonal character.

Example:

For instance, allow characters, regardless of their current level, to switch to a seasonal character.

The purpose behind this is to equip older characters with Tuvala gear.

This would be amazing.

Diablo 3 has the option to "rebirth" characters at level 1 at the beginning of a season.

It's not even Seasonal Servers anymore, since they don't graduate, Seasonal Servers really serve almost zero purpose.  

Just allow all characters to go on seasonal servers, but only allow them to equip Tuvala gear while on those servers.  Add to those servers tuvala lightstones and artifacts, Tuvala Crystals, Tuvala life skill gear (max pen loggia equivilant), and Tuvala Alchemy stones AKA a sharp alchemy stone recharged with Tuvala ore.  Then make all Tuvala gear character bound.

In other games Seasonal servers have an end time, it is a race to see how high you can get, now that there is no max there is no point.  

Add a character specific questing line that requires you to show Fughar pen versions of all gear, when done you get a "graduation ticket" which can be used to claim 1 seasonal reward.  Once you have all the current seasonal rewards, you can continue to do it for a 100 stack.  

Tuvala conversion tickets to convert 1 tuvala gear piece over to a real item that can be upgraded further will be given out with the graduation ticket.  Tuvala gear is armors and weapons only not accessories, life skill gear, or alchemy stones.