I definitely think the characters in the Stormcast list offer a pretty good starting point as a way to expand. It sounds like you've got a pretty smashy hand to hand force if you've got those starter sets. So you might actually want to think about a Lord Celestant on Foot. His command ability gives units in a 9" bubble +1 to hit. I've found it a bit more useful than the Lord Celestant on Dracoth's command ability, since I don't usually find my Stormcast failing Battleshock tests. I also like the Knight Heraldor, since he can blow up terrain with his magic horn, and he's got options for posing now too. You might also want to consider a Lord Castellant, who isn't a bad character, and his Lantern can either blast enemy units or buff friendly ones with a +1 to their save throws and a chance to regain lost wounds if you roll well. I like to use my Castellant to buff my Stardrake!
Judicators are a good choice if you want to spice things up with shooting options. I think the Knight Venator (Winged Guy with Bow and Bird) is a pretty great investment to boost those shooting/sniping options too. Also, if you have the Lord Relictors from the starting kits, you can have two of them placed into a Brotherhood of Great Bolts formation from the Godbeasts book. If you were to run the formation you'd need two Knight Vexillors (but you already have one, so grab another Banner Bearer or convert a Paladin using the banner you didn't use from the first Vexillor) and a Celestant Prime, which I would also say should be in the plans to pick up. The formation basically allows those four models (the Relictors and Vexillors) to chuck around mortal wound dealing lightning bolts (with a small chance of skaven-ish self explosion in the process) while also turbocharging the Celestant Prime's Ghal Maraz!
If you're going to dabble with Formations some more, you might also consider that you can use the contents of a starter box in a Thunderstrike Brotherhood, which allows the entire formation to 'lightning strike' onto the battlefield. Normally you'd have to keep those at around 9 inches distance from the enemies, making it a tough charge roll once they arrive. But if you have a Knight Azyros, you can deploy lightning strike models within 5 inches of him, ignoring the rule about being more than 9 inches away from the baddies!
So to summarize, I'd say you might want to consider a Lord Celestant on Foot, a Knight Heraldor, a Knight Venator, a Lord Castellant, a Celestant Prime, and perhaps a Knight Azyros. Beyond that, Judicators can be handy if you want more shooting, and the Stardrake if you want a colossal cool model that smashes face and acts like a bullet magnet.