What are the little green "+" symbols on the Arena Tier List?

I‘d say it means that these units could be a tier higher (under some circumstances or in general)

Edit: Guess I was wrong - I think we need a new tier or a mass demotion in the tier list, because the strength in tier 1 is very unequal and the units in tier 2 are really good compared to the weaker end of t1..

"A new icon has been added to distinguish heroes with unique support capability. Heroes with this icon - a green plus - are recommended as useful bonus carries and utility heroes." - taken from the patch notes