As someone who's been bringing sunshine on the rift for a long while, I play Leona top lane as a pure tank. Usually i start either Sunfire Cape (HP/armor/waveclear), or Thornmail (waveclears by letting minions suicide as they attack you) if their team is very AD heavy. Everything else is situational. Your team has a lot of AP? Get Abyssal, not only does it augment Thornmail and Sunfire Cape damage (and amp your own spells a bit), Just by being close to the enemy team you amplify all of your team's magic damage. Need a little more stickyness? Go for IBG. Want to tank mages? Get Warmogs and Spirit's Visage.
That said, i'm unsure how well Leona would fare played as a bruiser since you essentially waste the damage potential of most of your spells which ALL scale through AP.
I've found that Leona excels at a few things; killing/baiting foolish turret divers (CC Queen of League), annihilating autoattack-reliant champions due to massive Armor pool, Thornmail availability and potentially stunning them in your minion wave. (Tryndamere, Yi, Riven, etc), and receiving allied ganks. (CC Queen and passive damage is often underestimated.)
However, she has equal drawbacks; AD Casters and Juggernauts give her a hard time (Pantheon, Wukong, Yorick, Darius, Garen) since they usually can either outsustain you, outburst you or just shred your armor. AP toplaners (Maokai, Ryze, Ect) give Leona trouble because there's NO item that allows her to both waveclear and defend herself properly against AP, therefore you will most likely lose out on farm, and give the enemy a lead. Her AP scalings are too low for something like RoA or Rylai's to compensate in damage.
Then again, this is from the perspective of someone who plays her straight-on Tank. You may have more luck playing her in a more damaging playstyle than i have as Tank.
Let the Sun Shine on!
). And because most supports are squishy right now, as you say.
