I'm a pretty big fan of bungie, and have been so for a long long time. My experience with halo has me stuck as a titan for life. I have ideas for a 3rd subclass for both titan and warlock. I'm not a big hunter lover, so I'll have to skip them for now. These are just ideas, I sort of doubt you'll add more subclasses (which would be AWESEOOOMEEE if you did, but...)
These aren't fully tree'd out ideas, mind you
Solar Titan ideas:
First idea:
Juggernaut
Full on melee class. Shoulder charge using, big melee knife as a melee skill, and the grenade is just a storm fist equivalent, or vise versa. Super would either be a super mode, or a Kamehameha of sorts. Perks could be filled out, but would play much like an alternative to striker, but even more aggressive and risky. A super mode could have a perk that actually focuses on buffing allies.
Second idea:
Grenadier
Explained as the Porto-gunslinger from just after the fall, this supposedly rare class would allow a Titan to operate like so. It's otherwise a Striker, but is solar, and fires a Solar grenade launcher instead of a Golden Gun. Melee is a long-lunging knife attack. Grenades are probably identical to the defender.
Third idea:
Swordsman
Solar Sword of Crota Super, like a slow blade dancer. Note: blade dancer needs to be rebalanced a little. Grenades are like Defender, but the suppressor is replaced with a warlock melee or a melee-like effect. I love shoulder charge, but that might unbalance this class too much.
Fourth idea:
Deterrent (Conflagration?)
Super generates a field of fire in front of the Titan. Won't instant kill, but it's strong and wide. At least one grenade makes areas of fire. Melee is designed to catch enemies on fire or cause something like one of the storm fist perks where it creates that blast. Could be a long ranged melee strike like a "Solar Flare" This class would be made for keeping areas guarded via vicious force. A hobgoblin armor lock like perk for the super could be very fun.
Arc Warlock idea:
Only one idea for now:
The Summoner or The Cleric
Summons an attacking AI-boy like a Servitor. Initially it just sits there and shoots. One version follows, one version Romes around, and one version specifically fortifies. One grenade option is like the relic's cleanse, creates a temporary shield and ally healing zone.
The void hunter should be more of a passive-super like radiance and Ward of Dawn, maybe something like a vestige or a wizard poison cloud. Otherwise, I don't understand hunter well enough to describe a class.