I'm in line with Covenant here. Sunfire synergizes very well with Sejuani's MO: Either you're splitting or you're diving straight into the enemy team, and most of your damage (especially when building tanky) is already coming from your AoE. Granted, it's usually not the
first item I get after Golem, but it's pretty core on her as a straight tank.
If I'm going straight tank, my build is usually: Golem, Swiftness (mobis before the nerf--though I go with mercs if they have a lot of hard CC), Spirit Visage & Randuin's (order depends on who is fed), Sunfire, and either Banshee's or Warmog's depending on whether they have a high-impact ability that I'm going to need to eat for my team. As far as building damage Sej goes... if you pick Sej, your team should already have a ton of damage; you're there to dive the enemy team, CC them forever, and never die. I especially like to go with her when the top laner is a bruiser or a sorta-squishy fighter. Keep in mind, too, that with that build (finishing with Warmogs assumed), you've got 270 bonus damage on your W. Between that, Sunfire, and your percentage health on your Q, you've got plenty of damage with a tanky build.
Though I will admit that building damage/offtank Sej is incredibly entertaining to play. Those AP ratios are surprisingly good.
Mercs are unnecessary if they don't have a lot of hard CC (between Swiftness and your passive, slows are nothing), and Ninja Tabi are only necessary if they've got someone that goes heavy on AS/AD (as previously mentioned).
RE: Teambuilder. I wish that they'd open up the option of selecting a short pool of champions for the position (for example: Marksman Bot Lane: Lucian, Ezreal, Vayne). As it stands right now, there's no teamwork in building team comps. Either you get a good comp randomly, or you have the choice between leaving lobby because the comp doesn't work well or going in anyways with a comp with no synergy. One of the best parts of the normal system is that everyone can discuss who they're picking to set up a comp that works well together. :|