The set of features/abilities that sound fun to me are:
- Some sort of damage mitigation as a reaction (kind of like Protection fighting style, but reduces damage rather than causes misses?)
- Benefit to adjacent (nearby?) allies who are at 0 HP. (Advantage on Death Saves?)
- Improved recovery while resting, perhaps faster HD recover (no specific suggestions for how this would work...)
- THP to allies who actively participate, e.g. my suggestion in mellored's thread that a battle cry is started, and anybody who uses a bonus action to join in gets the THP. Clearly the benefit would have to be good enough to compensate for loss of bonus action.
A class that could do all that would help comrades survive in a lot of ways, none of which (unless I'm missing something) duplicate existing mechanics.
And a new idea for my "shield wall" concept: "When the (insert class name) uses the Dodge action, he can also use his bonus action and grant any allies within 5' the ability to take the Dodge action as a bonus action. Any allies who do so then grant the same ability to allies within 5' of them, and so on." (Needs some wordsmithing.) So if this guy uses his full Action to Dodge, everybody else in the group/line can also Dodge for just a bonus action, as long as they all participate. Maybe it's limited use, or takes Concentration, or something. The idea is that he's not literally taking the Dodge action; he's using his Action to oversee a defensive formation, and then using his own bonus action to Dodge.
Maybe it's complicated, but no worse than Hiding rules!