They could always publish more spells, those always seem to be easy to make
When they tried to make the Mystic they ended up crying to cram all 4e roles into a single class. If they just focused on a few aspect they could make it to work... like, we don't need a Psion to get the melee psionic stuff, we got two Psionic subclasses for that.
I could see a Plant Druid (folded into the Farmer Druid I think, Circle of Harvests?) and a Whirlwind Barbarians being good future subclasses, same for the Dragon Warlock. Cleric domains are also an easy possibility. A Brutal Thug Rogue who gets bonuses from STR and can use clubs to sneak attack would be neat. Or maybe as a Fighter subclass with a Rogue MC feel to it?
Personally, I'd like to see:
1- Warden Barbarian that transforms into embodiments of natural phenomena when raging
2- Gourmet Bard
3- Chaos Sorcerer as a retry on the Wild Magic Sorcerer
4- Retry of the Giant Blood Sorcerer
5- Some sort of Loreseeker/Tomb Raider Wizard, a guy who specializes in exploring old ruins to find magical secrets
6- College of Masks Bard, actors who can 'become' somebody else (basically modular skills/level of combat ability)
7- Some sort of Steel Monk who, while retaining DEX as a main stat, can make use of armour as an offensive tool
8- Some kind of class that use the Chinese 5 elements would be cool. Maybe a Wu Xing Monk to replace the 4 element one and an Onmyodo Druid?
9- Or a Monk of the
Four Symbols, or even use that for a Paladin or Fighter if we don't want to slap all the Asian concepts into the Monk.
10- Some way to bring back the 4e Avenger.