Exactly what you're looking for...
One phrase which I believe will help you is 「~と同【おな】じくらい~」. This literally means "to the same degree as" and seems like it will fit what you're asking for. Some examples:
- 耳【みみ】が顔【かお】と同じくらい大きい。 His ears are as big as his face.
- モデルと同じくらいかわいい。 She is as cute as a model.
This form will with with i-adjectives, but with na-adjectives, you'll need to use に, e.g.:
- ちっちゃい魚【さかな】もおっきい魚と同じくらいに[上手]{じょうず}に泳【およ】ぐよ。 Small fish swim just as well as big fish.
...And a [better?] alternative
Of course, you might also consider using 「~のように~」. It doesn't capture exactly the same nuance, but I think it does well enough. It also has the benefit of sounding much more natural in many scenarios. For example:
- 輝【かがや】く瞳【ひとみ】は星【ほし】のようにきれい。 Your shining eyes are beautiful like the stars.
Extending the meaning
Both of these phrases are equally versatile when you want to extend their meanings. You mentioned "not as", "can/might/will be as", and "want to be as"... all of these are possible by doing what you'd normally do in Japanese to say these things.
- モデルと同じくらいかわいくなりたい。 I want to be as cute as a model. [Note: It's true! I do!]
- 耳【みみ】が顔【かお】と同じくらい大きくなるだろう。 His eyes will probably be as big as his face.
- 人【ひと】は魚と同じくらいに[上手]{じょうず}に泳【およ】げないよ。 A person can't swim as well as a fish.
And just in case you really want your girlfriend to break up with you:
- 瞳【ひとみ】は星【ほし】のようにきれいじゃない。 Your eyes are not beautiful like the stars.
Note: basis for example sentences sourced from various Japanese-language websites.