If you recall back to your lowbie days you will remember that many quests in the harbor such as Butcher's Path and Water Works were run with full groups. Why? Simple. Secret doors, bashable doors, strength levers, rune checks, traps, etc.
Multiple types of these elements within the same quest encourage grouping with varied party members regardless of whether you can actually solo it skill wise or not. Another good example of this the Lord of Dust series which there are always LFMS for. Difficulty asside, the occurance of traps you can only disable from the other side encourages rogues, however the pressence of undead which can not be sneak attacked encourages the rogue to be accompanied, cleansable alters means lets find a pally or cleric, that extreme difficulty challenge means we need a caster or bard for crowd control, and so on.
This is how you correctly encourgae grouping.
By the way, these quests do not actually prevent you from running solo, or short on them such as needing to hit simultaneous levers or split up or anything, they just give you an incentive to group up and reward you should you choose to.
My suggestions as I have said before would be to expand this line of thinking with new and varied uses for the Arcane, Relegious, and Wilderness lore functions (like you said you were gonna do before you got distracted making daily dice and astral shards) as well as making race specific route obsticles: such as small tunnels or vents only halflings can fit through to unlock the door to let the group through... or vacum rooms which can only be passed through by warforged or necromancers in form. It would also be brilliant to design at least one underground cavern maze with DM hints given to dwarves as to direction and slope similar to the hints that pop up if you have a high spot or listen.
As a DM for two decades I can tell you that whether players feel like they need to be in a group or not is mainly dependent on whether they feel like the setting gives them a reason to. The actual setting, i.e. the feeling of the quest, not neccessarily the game mechanics needed to complete it.