For ages I've wanted to design another "Form of the" card and Form of the Dinosaur reminded me of it.

I think the secret is that in order to feel powerful, the card needs to protect you to a significant extent, as well as making an active difference in some way. Dragon does that by preventing non-flying creatures from attacking you and resetting your life each turn. Dinosaur by setting your life total to 15 to start with.

But another card needs to find a different ability, as they can't ALL be "at the beginning of your turn, your life total becomes 5". I looked at many iconic creatures before morphling occurred to me. You don't exactly have power and toughness, but it lets you trade life for library or vice versa.

I'm not sure how good is too good when it's giving blue potentially unlimited lifegain.

The hexproof seemed like a good fit as well.

I toyed with abilities that felt like untapping yourself, or blinking yourself, but nothing felt right :)

Huh, clever interpretation of morphling. Resource conversion like that definitely makes me want to break the engine, but probably pretty convoluted. Note that both abilities unmill right now, so that's a typo.