Why not a new talent tree available after lvl 30 that will let you play with variaty of options.
These are just quick thoughts that will keep the old classes the same in a lot of ways and will add some extra fun…
for example:
Warrior will have 2 new talent trees available after level 30 - the old talents will remain the same or will be balanced to keep up with the new ones - also the bonuses wont be just for new shields (in this case), etc… they will apply on the shields we have now…
Guardian - new shield with talents to add… i dont know maybe this: no damage for swords bonus, ability to deal area damage instead of stun after 6 blocks without taking a damage + for all shields passive small hp regeneration per block (eg based on % of monster damage) … special will idk reset shield
Frenzy bastard - ability to use 2 swords with lowered damage per sword, updated special double combos with using 2 swords, the lower your hp gets the more lifesteal you get… special will add you %dmg and lifesteal for X seconds
These are just thoughts from last few seconds while writting this so dont judge them because im talking about the idea of keeping the old classes (probably less coding and messing around with completely new class) and updating their talent system to basicaly upgrade them with some new mechanics OR just new bonuses to the ones we have now… warrior will still be a tank with the old combos etc… but will get a possibility to use a new shield and a bit different playstyle OR will use a fun (maybe?) to play mechanics with 2 swords but being squishy…