I would rather not have her proc her own passive, simply put the damage it brings to allies helps them secure kills, not Leona. In fact, the centric idea of Leona was to amplify her allies damage and 'suppress' the foes as much as possible, and she is so very good at this...
You do make a good point, her passive on her own does nothing, and in respect to your ideas, I agree that something should be done that does not take away from the power that the passive does to her allies, but makes it beneficial to herself, so I propose something along the lines of the following:
- If any sunlight passives wear off enemies, she gains an increased health regeneration buff that refreshes upon other 'Sunlight's fading off foes
(This will immensely help her in lane for substain, she has the tankiness and presence but most tanks like Alistar and Shen have some sort of ability that keeps them around their ally, this change helps her heal off minions, champions, whatever)
- If any sunlight passives wear off, her cooldown on her Eclipse is reduced by 0.5 seconds each different passive fade
(Different from substain, simply put, this will increase the uptime on her bubble in all stages, should enemies not have their passives removed by Leona's Allies, she can keep up her bubble, which is her strongest AoE damage spell, up)
The above suggestions are some examples of how to make her passive partially useful to Leona on her own or whatever.
ANOTHER big concern, that most Leona players hate is the overlapping passives.
There is No Way for any Leona player in any battle situation, to perfectly set up all four passive procs of her abilities on a single target when she goes in with her teammates, conditions of battle typically only allow 2 and if you are lucky, 3 procs of the passive since you are shifting between stunning, bubbling, snaring, etc.
I would like it if the passives would STACK on each other on a target, still lasting 3.5 seconds of course, but at least when your AD carry is focusing that target, the passives would each proc again and again, enabling the full effect of her passive.
Personally, if they made her passive STACK, that would be good enough for me, the damage buff would be AMAZING in team-fights, and just what she would need to put aside any doubts of her actually needing a passive that helps her.