Leona needs higher damage output, her tank and CC are fine, her CC is only rivaled by a few champs in the game, but the bigger problem is that she lacks any real damage for end game.
Some builds for tanky bruiser Leona are:
Merc Treads, Wits End, MBR, Cloak of flames, abysmal, (last item can be anything that adds lots of hp/hp regen like warmogs). This is very expensive for Leona to build - provides for decent tank (not the best) and good dps as everything works off the synergy of the abysmal scepter and her W skill, with 9/21 masteries ( to include CD in both offensive and defensive and the 10% magic pen in offensive) and going with per level health quints, armor seals, mr glyphs and flat magic pen marks. This build works best with AD heavy teams.
(2) Merc Treads, Warmogs, Atmas, Maw, Trinity, GA or Aegis. This build requires you go more glassy offtank, 21/9 runes, using the physical attack side of the offensive tree, 2 each in armor and mr, 4 in hp, 1 in 3rd row hp on the defensive side. Runes would be pretty much the same, change out the magic pen marks for dual pen marks. This build works well for AP heavy teams.
I have yet to come up with a decent hybrid damage build, they all end up way to glass for an offtank setup.
As you notice - both builds that provide any type of damage output also cripple your CDR, which means you have to less overall tank ability due to the fact that your W Skill will not be up as often nor will you be able to stun as often. Both builds are extremely expensive for Leona and are normally not attainable without hurting your carries or jungle by taking valuable minion kills from them.
Overall Leona is my favorite tank champion, but realistically, even Sej is more viable as a tank than Leona. I can easily out tank and out damage Leona with a mana build Taric and provide just as much, if not more utility for the team.