I was building a volc team, so I started with volc. From there, I thought about the things that volc has trouble dealing with, namely kangaskhan, heatran, talonflame, diancie, landot, and keldeo. I wanted checks to all these things.
First, I put Lucario on as a check to Kangaskhan, Diancie and Heatran and as general follow me support for volcarona.
From here, I was looking rather weak to Talonflame and Landot. I added Deoxys-Attack as a more proactive Landot answer, because Deo-A is an offensive check rather than a defensive check like washtom, and thus can kill Landorus-Therian before it gets a chance to rock slide.
I still wanted better Talonflame answers than a Deo A revenge-kill, so I asked around for some help. Some faster-than-talon priority moves that are good include Greninja's Water Shuriken, which someone suggested. It does around 75% with 3 hits and is a guaranteed OHKO after brave bird recoil, usually (it isn't if you've taken TOO much life orb recoil). In addition, if it gets 4 hits or above it'll obviously KO, which is a neat set.
Looking at the composition: Volcaron, Lucario, Deoxys-Attack, Greninja... I was rather weak to those bulky tailwind/speed control abusers (ie bulky Suicune, Zapdos, bulky Thundurus) so I went for an option that is usually capable of taking advantage of those mons, Breloom. While it does stack fighting coverage, I felt Breloom was also a solid pick as a water switchin and a good choice that could spore or ko bulky attackers that would have given the team issues. In addition, Breloom is a fantastic trick room check. Breloom offers the ability for this team to look at slower compositions and to just shake its head while Spore'ing everything.
The last slot on the team went to Togekiss, which has excellent synergy with Breloom (typing and tailwind) while also having nice synergy with Lucario's Steel typing. It is kind of weird to have 2 users of Follow Me on this team, but I think I like it because it allows me to setup on Volcarona much easier and also Follow Me is a great move. Also Togekiss is a really nice stop to Talonflame Brave Birds. Follow Lucario is kind of wonky but I felt most of the other options were rather poor too, and it's great as a surprise redirection (especially since lucario's typing lets it tank many things the other pokemon on the team do no want to take) and for stopping Kangaskhan Sucker Punch on Deoxys-Attack.
Breloom is life orb because I have so much redirection I felt I could get away with being greedy for damage. Life Orb is a cool set that lets me be less afraid of Fake Out and switching in on chip damages like Rock Slide or weak water moves, and is relatively unaffected by Hail/Sand.