Suggestions for Trollbloods as a 2nd Faction

If I was going to play Trollbloods on the cheap I'd go all (or at least mostly) beasts. eDoomy is probably one of our most competitive casters right now, and Horgle or Ragnor with a beast brick can compliment him well. You'd need a Krielstone, Runebearer, and I personally like Rune Shapers in the Dhunia theme with both Horgle and Ragnor. You mentioned Calandra, she can also run a beast brick in Dhunia quite well.

I would say Grim2 is our best ranged caster. Mortality is amazing and his feat turns our ranged options up to a respectable level.
Gunnbjorn is probably second. Snipe is awesome and his wall is great for control. His feat can be hit or miss.
Calandra is a good all around caster. She has strong control aspects and can skew dice to the extreme.

Definitely magnetize the heavy kits so you can go mauler or bomber as appropriate- or make one of each and magnetize a third, that works well, too, and you're going to use both the mauler and the bomber enough that it's worth dedicating a model to them.

Calandra is absolutely excellent in most lists; a good rule of thumb that has yet to fail me is that if a DT Bomber is in range of the enemy 'caster on Calandra's feat turn (especially if it can aim), and they aren't camping a stack of focus/fury for overcharging or transferring, the game is over. She also supports infantry well with her excellent spell list. In truth, she can run almost anything- because she messes with the dice, she doesn't have to favor any particular build. They all use dice, after all!

For range, I second the previous opinions about Grissel2. If you haven't tried her yet, she's lights-out great, even in spite of her terrible feat. I always, always field the Runebearer with her (heck, I do with almost everybody except Borka)- she's a very fury-greedy caster who struggles to feed the Krielstone otherwise.

If you're looking to go beast-heavy melee, Doomy2 is definitely your guy, though both Ragnor and Horgle2 can do this as well, though they tend to be a little more "take the hit and THEN kill your opponent" than the much more aggressive Doomy2. If you think about branching out into infantry, consider Trollkin Champions with Skaldi Bonehammer and run them with Borka Kegslayer. They hit hard, the guy gets them up to an absolutely absurd defense and targeted dispels like Hex Blast don't work on them. Just don't drop them into anyone who's running models with Purgation.

Good luck! Welcome to the United Kriels. =)