I am not really sure how this would work. Something like a trident is there, so why not get a trident? Forging is not really a creative process, since those combinations also need to be allowed for in advance. So you might just as well create new cards.
What can forging do?
1) Create cards we already have outside their element? If so, you would be able to build very strong mono decks. We no longer need different elements.
2) If not, does it stack abilities of one element on one card? Invites imbalances as well.
3) If not, does it simply take two sources to obtain a card we already have in the game? If so, why don't you just get that card? The idea of failed forging is not appealing to me at all, btw.
There is not a way of forging that has added value, if you ask me.