Alternatives to the biochemistry of Earth life are hard to imagine. Silicon replacing carbon is one possibility but, as yet, despite silicon being vastly more common than carbon, we know there is no silicon life on Earth. It's just at too much of an inherent disadvantage, if it is even possible. I would bet on panspermia over silicon life.
Imagining an alien intelligence is easier than imagining alternative biochemistry. Odds are looking relatively good that we may meet an alien intelligence by building one.
