I think Frontier, as a format, is in good health. The daily wheel, the new Vanguard cards, and the chest quests keep its appeal intact (opening packs is always fun).
As for the meta, I find it hard to see it becoming unbalanced. In theory, if you play consistently, you'll eventually catch up to your opponents' level while building a solid foundation to make the leap to other formats like Modern.
Getting back to your question about what I miss, it would be fun to imagine a card that combines all my favorite abilities. While I understand that the biggest challenge for any designer is maintaining balance, dreaming is free. Without getting into stats, I don't know if abilities like GIANT KILLER or LAST STAND would break the game, but I'd love to see something like that in Vanguard Arena. And if its art were also a tribute to the movie Highlander, I'd have the perfect excuse to yell before every battle... "There can be only one!"