On the card idea, I'd have to say that immortal/immaterial/untargetable is extremely OP at the moment, anyone can attest to this if you look at Oty, Leaf Dragon, Lava Golem, any creature with a great built-in ability (including Anubis, Nymphs, etc.). While most of these take a long time to get going, just look at Oty in a rainbow deck...once quinted, the only chance most opponents have is creature rushing, unless you have poison or fire shield. Just my opinion, as everyone is allowed their own view.
As for card ideas, I get what Kael is saying, but the only problem with that is that it stifles 99% of the community, when you only need/want to stifle the 10% who over-post...I myself love making new card ideas, but have no extra time to put forth into testing, so you would lose every card I think of. As far as knowing mechanics and such, I've played CCGs for over 15 years, so I have an idea of how a card would change the game without actually playing it. In fact, for most CCG developers, this is exactly how they design new cards...they sit around a table and give their ideas, then brainstorm how to make them better/less overpowered, then playtest to see how it actually works. Most of the time, after 5-10 people look over a new card, it can be seen without playtesting what mechanics of the card need to be tweaked, or if the card is actually possible at all.