You still have the problem of conquering planets you can't use in Galciv 2 because the other race's technology is more advanced than you when they colonised it. I don't think the argument is valid and having different races use different types of planets in the beginning would make the game more interesting. There will still be common planets that everyone wants for one reason or another and as techs are researched, the playing field is leveled out more and more.

You could have planets with more than one attribute eg: A planet that is desert, volcanic and toxic. One race thrives on desert planets, another on volcanic, and another on toxic so the planet will appeal to all three races at first. As more research is done, other races will also want access to that planet.

Honestly, that's the best damn idea I've heard about colonization in a long time, very interesting.!

Of course, I can see some problems; 

1. Races that prefer rarer environments will be at a severe early-game disadvantage compared to others which prefer other, common environments unless they had a massive advantage on those planets they could use. 

2. If every planet is colonizable and useful in the late game, we'll have a fuster cluck of planet management in the late game on big maps, like in GCII on immense maps.

3. If every race can eventually colonize every planet, then it removes a huge part of the uniqueness of each race

It never has to completely level out in the end; each race's uniqueness could always keep them superior. If every race could, for instance, research up to +30 for a planet attribute by the end of the tech tree, it will still leave the race that started at +20 with +50, while races that started with 0 would only end up with +30.

I'm against the idea of a linear tech tree that has an end BTW. I'm all for infinite research, where you can always research a +1 or more to something, at exponentially increasing costs and time, but that's for another thread such as this one: https://forums.galciv3.com/450312

i kind of like this idea but possibly make it linked to other worlds for example i have 3 earth like worlds say PQ 10 and i can gain +15 for each of those through research but i lose 5 for being an aquatic race for a total PQ of 60

(assume that an earth like planet is normal and each planet type is 1 step off of normal

so if you like volcanic then earth like is -5 and aquatic is -10

if you like earth like then every other type is -5

if you choose a type other then earth like you get a +10 to that type and an additional +10 from research

earth like planet's are more common and are "closer" to all other types so don't get a bonus)

lets assume the planets are relatively close together and i drain 2 of the planets and dump all that excess water onto the third planet. changing the first two planets to barren and the third planet to aquatic

now i have 2 barren planets which effects me at -10 due to my preference for aquatic worlds  so they are PQ 0 and one which is aquatic giving me +10 but i can enhance the aquatic by another +25  so now i have 1 planet with an effective PQ of 45 and 2 planets which i could research up to 15 granting me a total PQ of 75 or i could turn the two planets into mining colonies which might give my third planet some manufacturing bonus. or i could try and further terraform one of them into a volcanic world and give it to an allied race(who happens to prefer volcanic worlds) in exchange for their new battleship

I like this idea. Also, you could get military bonuses when invading/defending a planet which is of your preferred type.

Maybe you can also have a separate "favored terrain" attribute - plains, forests, mountains, etc - affecting how much use you get from the different continents on the planet. Again, this would affect planet defense.

In the game Stars!, a planet has three environmental values - gravity, temperature, radiation - and at the start, you specify the range your race can live in. You can have your species be able to live ANYWHERE - or you can have them be super finicky and get a ton of extra points to spend on other race attributes. You can also terraform planets to fit your race, but changing a planet too much outside of its natural value is pretty hard, so some planets remain more attractive than others. 

Having different civilizations prefer different planets can be a feature - another variable to consider. If you make an ally of a civ that loves planets you hate, you would have much less competition, and together you could make the best use of your territories. On the other hand a civ that shares your environment preferences can interact with you a whole lot easier, fostering good relations, increased trade profits - and more spying opportunities. 

This also bothered me about GalCiv2.  A class 20 planet was a class 20 planet was a class 20 planet.  It didn't matter what race you were playing.

The Torians should be aquatic and the Yor should be able to colonize a larger array of planets because they're robotic.  The real question is how to balance this?  I assume pay more race points for being able to colonize a wider variety of planets and less (negative) for being able to colonize only a small section of planets without terraforming.

A planet's class should matter on your race.