Are there any replacement keyboard apps?
Sorry, but currently, no. Jailbreaking is your only bet (I'd advise against this, though). 

In terms of what you want, it sounds like you are describing something that iOS already does. When holding down on the keys, it "magnifies" the key that you are holding so you can see what you are typing. I should mention though that holding down on certain keys will show you options of the same letter with the accents on top. Ex: Holding down the "E" key will give you options such as "E", "È", "É", and "Ę".

Here is an example of the action described above, but with a different option