Here’s what I’d do:
New classes every 2 class ranks instead of every class rank, I.E. R2/R4/R6/R8/R10 (with R10 classes being single-circle on par with R8 C3) so that power differences between class ranks are less drastic. Ex: R2 Cleric would be Priest, Diev, Bokor, Krivis, R4 would add Sadhu, Pally, Monk, Pardoner, so you could go Cleric -> Diev2 -> Sadhu or Cleric3 -> Monk, etc.
R8 prestige classes that require C3 of 2 low rank classes, different from hidden classes because they also have 3 circles (and also aren’t hidden), adding new builds from uncommon low rank class combos that would be very specialized around those classes’ common elements.
Ex:
Quarrel Shooter + Sapper = a defensive area denial class focused on placing barricades and traps while doing basic attack damage
Ranger + Hunter = an aggressive class with combination attacks increasing damage on a target when you and your companion focus it.
Priest + Krivis = maximum overbuff class for when you just want to dump a million buffs on your party and have them last forever.
Bokor + Dievdirbys = crafts dark idols that debuff/damage enemies and buff your zombies.
Highlander + Barbarian = berserker class that reduces sacrifices defense and inflicts self-damage to chop things up extra good.
Hoplite + Peltasta = tanky spear/shield class with an emphasis on blocking and counterattacking + killing large/elite/boss enemies.
Pyro + Cryo = just Elementalist because it fits so well. Add new earth mage class to replace Ele. TFW 2017 and no earth mage.
Linker + Kino = has stronger links that can link more allies and don’t break when allies are too far apart, traps linked enemies together so they’re pulled back in if they drift too far apart, gathers up groups of enemies for AoEs, basically the class that will ruin GVG forever.
