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Update 7.1 Arena Changes
https://us.battle.net/forums/en/hearthstone/topic/20753189941#1
Summary of the changes
Arena is changed to Standard format, Blizzard will monitor that, might see some other format changes in the future.
Card rarity distribution modified (Commons less common, rare/epic/legendary more common).
Spells will be more common.
Less neutral cards showing up to avoid having to pick another bloodfen raptor.
Class specific changes : Abyssal Enforcer and Flamestrike will be offered less often.
Blizzard just posted those changes that should be coming in 7.1 and will affect Arena in a pretty huge way. Idk how many competitive arena players there are on this subreddit but I'd be interested to see discussion about these changes. Specifically the spells changes (more AoEs potentially ? some classes with pretty bad spells like Warrior could be impacted negatively tho if one of your epic choices is a shield slam ?), and lowered occurence of Flamestrike and Abyssal Enforcer (which goes in the opposite direction, but felt needed to get some class balance back).
Edit : a comment below clarified the offering changes : https://www.reddit.com/r/CompetitiveHS/comments/5vjnn4/update_71_arena_changes/de309mq
Iksar
We thought about rotating sets in that weren't just standard, but went with standard mostly do to simplicity since there were so many other changes going in at the same time. The amount of work was never a concern. There is very little difference in terms of work effort between making standard arena vs some other combination of sets. We'd have to figure out how to only make the combination of sets we want appear from things like Babbling Book or Webspinner, but I'm sure that would be a solvable problem.