Depends on your play style. I have a Vampires deck that is really fun, tons of spot removal, and
Vampire Nighthawk with 1 or 2 Stromkirk Captains is killer. I'm B/R/W to splash
Sorin, Lord of Innistrad, which has tons of synergy, of course.
I've been wanting to make a Mono-blue Faeries deck for Modern, but with Theros out, I'm thinking about doing Merfolk instead and going with 3-of Thassa.
If you're playing legacy cards, Clerics can be killer, since there's a lot of combo potential there.
For White Weenies or White Agro, Soldiers have a lot of synergy. but as a challenge you could try to build a good Knights deck. They're a tribe, but there aren't as many options as a lot of the other tribes, so it's a bit harder to build. There are still a lot of good choices though.
Pretty much anything with a Lord (the cards that give +1/+1 to the others of the same tribe) is do-able, or even without there's still
Coat of Arms.