The first expansion I ever wrote was Dragons of Tarkir. I love Tarkir and the flavor really lent itself to this game.
The second expansion, Khans of Tarkir (fate reforged, of course, gotta stay canon folks), was easier to make due to a clearer flavor. And yes. You can play them against the dragons, though they have one unique mechanic not possessed by dragons. I'm trying to make every expansion unique-ish and the Khans needed something to power them up a touch if they actually wanted to stand a chance.
Expansion number three is Allies vs. Eldrazi. This expansion simulates the fierce fight on Zendikar but you can also take the Eldrazi to another plane. How will they stack up? Only hours of playing will tell! Or one game if you're not a NERD like me.
Expansion four is one of my favorites: the Shards of Alara. This is, I suppose, two expansions: pre-and-post-Conflux Alara. Once pre-Conflux Alara loses half its base life, everything on the battlefield is destroyed. You play the rest of the game with post-Conflux alara creatures, which are similar but their stats are shifted to be a touch more warlike and each shard gets a few creatures from different shards.
There are, of course, more expansions coming, I actually have a list of all the ones that I want to brew. I am excited for the future of this game and I hope you are too.