It's very difficult to find an island that has all current biomes with both meadows and mistlands. Almost impossible if you say it can't be a huge continent.

You can find them with blackforest up to mistlands though. I'm currently playing on one, no bosses on it though. But having all metals in walkable distance is nice.

You'd pretty much need to keep a list of suitable seeds because just finding one would be a challenge all by itself.