Before Skyhold, I wouldn't even think in terms of a "build." You're not looking for synergies so much as grabbing the individually best skills. The only exception you might need to worry about is mana management. If you have more than two damage spells or your damage spells are both mana-hogs, you will need to take at least one mana passive.
The folks here have all pointed you in generally the right direction. Barrier, Fade Step, Immolate, and Winter's Grasp are all great skills. Dispel is good if you have room, but you only need one on your team, so if you have two mages I'd stick it on your follower. Consider disabling the AI from using it, or they'll spam it constantly to proc combos. This can be bad because it breaks your cc, prevents awesome ruptures and shatters, and means you won't have it up when you need it. Fortunately, your pre-skyhold team probably doesn't have any sleeps and might not have any stuns, so this could be a non-issue. Now I wanna pick on a few specifics.
Unlike several posters here, I don't think upgrading fade step is really worth it before skyhold. It's an awesome escape skill, sure. The actual attack on Haven is way, way easier if you have it. (In fact, I often pick it up as my last skill before Haven) The passive adds damage with no mana cost, which is nice. But the total DPS is pretty bad, and if you're using it for damage you're not taking full advantage of the no-move passive. I would always take that over upgraded 'step. I would also upgrade Immolate, Chain Lightning, or Winter's Grasp before Fade Step. If you have Fire Mine, upgrading that is also a priority. It's not until I'm looking at options like upgraded barrier, threat reduction passive, or upgraded energy barrage/lightning bolt/ice mine that I consider upgraded fade step viable.
Unlike Nathanaws, I don't see the point of a "fire build" at this point in the game. Immolate is incredibly good, but that's as far as I would go. One passive gives you free cooldowns on critical hits, but you won't normally have any crit chance at this point in the game. One gives you free cooldowns for casting spells. That's good, but not as good as just adding another spell. Both increase your hunger for mana, and you won't have enough points to take both the cooldown passives and the mana passives yet. Wall of fire is great but may not be worth the cost in ability points.
Naclyn's advice on specs is of course irrelevant before skyhold. He's right about Rift Mage; it's amazing. I don't think K-E is as hard as he thinks it is. You have a pretty low-cooldown invulnerability spell and you can get multiple sources of cooldown reduction. Before Skyhold, I agree that fire/ice works really well. Immolate is amazing in its own right, and while Winter Grasp is good-not-great, it opens up amazing passives. This is what I mean about having less a "build" than a pile of good stuff. However, I would skip Revive. It's not that hard to revive allies by hand, especially if you have fade step. Revive does it faster, but it also uses an absurd amount of mana. Basically, it prevents you from losing several seconds of autoattacks but at the cost of your next couple nuke spells. That's a net benefit but not a big one, and if your mana isn't nearly full when your ally goes down, then you're leaving them dead for longer than if you just rezzed manually.
As a final note, remember that even in an end-game scenario, there's little reason to specialize in one tree. Stacking the passive skill from across the element trees is a very good idea. Unlike in Dragon Age 2, there's no +%cold damage or +%lightning damage items. Cold has one passive that pumps up cold spells, but it's not good. Fire has two, but the barrier consumption one works great with just one fire spell. Getting Immolate, Wall of Fire, and +burn passive together is pretty good, I'll admit. But so is Immolate/Horrify/+burn if you're necro, or Immolate/any rift mage passive/+burn if you're a rift mage. As far as I know there is no storm passive that specifically enhances storm spells.
The only other effect that rewards specialization in one element is energy barrage, but unless your element is lightning, then you're crossing trees by definition to pick it up. I don't recommend focusing on lightning because while the individual spells are good, as a group they have some severe weaknesses. Lightning Bolt and Static Cage are both worthless against paralyze-immune enemies. Chain Lightning is crap once there's only one enemy left. Everything is crap vs. red templars. Those 3 problems tend to stack up. If the you're fighting a boss with paralyze-immunity, then you become useless as soon as you run out of minions to bounce lightning off of. If you're fighting a boss with lightning resist, you're useless immediately.