IMO elements does not need new cards now. New cards are not enough to draw in new players. If you aren't playing the game, why care?
What it needs is a new game mechanic. Some non-grindy mechanic. Probably it can be renamed as elements 2.0 and be re-featured in Kong as said before.
The new mechanic should be PvE as PvE content is resistant to gradual loss of playerbase over time (unlike the silent pvp2...).
The problem of EtG is not "not enough cards", it is that only 30-40% of the cards are used competitively. If there is some PvE content that utilise those "unused" cards in a better way (unlike the oracle deck which forces ghostmares/oraclebows mostly) and completely revolutionise the gameplay, elements can sustain its place.
As for PvP, there is a lot of potential besides random matchups. However, if PvP is to be improved first, the main PvP content should begin to detach from the forums. You may argue that the forums are very accessible, but some might be too "shy" (ran out of vocab) to enter the community or wants to play solo only. Even to me, sometimes, the forums feels like an intangible, elitist circle. Why not open the opportunities for the non-forum-goers? It could be said that they give up their opportunities by not signing up, but signing up is somehow a barrier to them.
tl;dr: my signature.