If I was doing a card game with 6 suits, I wouldn't use the traditional 4 and add two more. I'd design six from scratch. I'd use six different colours, and not use symbols at all.
However, if I was to answer your question as opposed to what I think you should really be asking, I'd use something like: red diamonds red hearts black spades black clubs gold fleurs-de-lis gold acorns
German decks use hearts, bells, leaves, and acorns. There's also the Tarot suits of cups, coins, wands, and swords if you're after more traditional symbols.
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However, if I was to answer your question as opposed to what I think you should really be asking, I'd use something like:
red diamonds
red hearts
black spades
black clubs
gold fleurs-de-lis
gold acorns
German decks use hearts, bells, leaves, and acorns. There's also the Tarot suits of cups, coins, wands, and swords if you're after more traditional symbols.