Gas Giants are usable. All you need is a bit of AG and radiation shielding.
For instance, there's a range on Jupiter, where, if you could manage to not fall into the planet, you'd find a mix of gases that are close enough to support Earth life. A human wouldn't be able to survive the exact conditions unprotected, but much of the simpler life here on Earth could. Toss in some Anti-Gravity and some decent radiation shielding, and you'd be able to "colonize" Jupiter just fine: think cloud cities of Vespin. They could have ships that go out and harvest hydrogen to bring back to power their city's fusion reactors (providing cheap and plentiful power to run all that AG and radiation shielding).
There isn't any place but a star or its more massive relations that cannot be reasonably colonized by an adequately advanced technological star-faring civilization. There's just a question of whether there is any motivation to build something there.
So, the real question is: Would that be fun? Dealing with planets isn't fun in GC2. It's just resource management. You do it, but its a drag on the enjoyment of the game. So I don't think we should be putting forth the idea that we should be adding significantly to the amount of colonizable worlds... at least, not until worlds are as easy to deal with in GC2 as they were in GC1.
As for being able planet class and colonizing... as you increase in your tech, you should automatically gain tiles. This means in the beginning, a "Venus" world would be class zero. But as you gain material and construction techs, it would become "Class 1" and "Class 2". As you continue to gain techs, it would slowly grow, until it was effectively a nice little spot in your empire. Of course, Earth would be an over-grown monster by then, as techs would also improve those few "perfect home-world like" planets. After all, if you can build a domed city to survive the pressures and corrosiveness of Venus' atmosphere, you'd certainly be able to properly build in and more fully utilize those "useless" deep ocean tiles on Earth.
This is entirely logical. However, this would be less fun then the current system, as it would mean that you'd end up having to give yet even more build orders at different times to your worlds (all those new tiles!). So, I think the critical thing to do to the game is to concentrate on reducing the micro-management of the worlds as they currently stand, before adding in "gaining the use of more tiles/worlds". Otherwise, it would detract, rather then add, to our gaming fun and the flow of the game.