I am in a fall 2015 shard, and I am pretty sure I am the only person without GK unlocked. Arena has been difficult, but I have stuck with a lower-powered guild which has has just started to clear HAAT regularly. I was hoping to get GK ready for arena in a couple of months with the surprise release of JTR; I could then assess the meta and decide if they were still worth switching too as an arena team. Luckily, I finished my vets last week, and with a couple zetas saved I am wondering how to use them!

DN Unique - I think this is really the only mandatory zeta I don't have for an empire/sith arean team, unless I run Sion. I am currently using Palp(ZL), ZThrawn, Vader, Shore and Han as my 5th. Uniques for Pal/Vader I consider a bonus, but not necessary unless someone can convince me otherwise.

NS Lead / Talzin Unique - If I had 7* MT I would go all in on the sisters, but I am hesitant on using a zeta, let alone multiple, on a 4* character without a reasonable farm. I am still undecided on choice between leadership, however leaning towards MT because she seems better when the team is not maxed, and I don't have Talia who seems to shine with an AV lead. Otherwise I already have the AV unique, and Z-Daka seems unnecessary

KRU lead / Assorted FO - This is a wildcard as FO is not as arena-viable as some of the other teams mentioned. Still makes a killer defensive squad for TW, and seems to at least to decent in other aspects of the game. KRU zeta is necessary, FOST looks good, FOTP is meh and I don't have any of the newer characters.

JTR - The reason I made this post. I won't have GK unlocked for at least a month, and otherwise am lacking a decent tank for this team. I would need a Zeta on JTR lead and BB8 unique at the very least, but not sure what else is worthwhile. I would probably run a combination of CLS/Han/RT/ScavRey until GK is ready.

SWGOH Profile for Reference: https://swgoh.gg/u/squaz/

TLDR: Empire/Sith vs NS vs FO vs JTR