I know moral relativism is not universally popular, but can reductionism/rationalism lead to anything else?

That depends a lot on what you mean by "moral relativism". Certainly rationality and reductionism need not imply taking morality less seriously. I liked what Eliezer's Harry Potter had to say on the subject:

"No," Professor Quirrell said. His fingers rubbed the bridge of his nose. "I don't think that's quite what I was trying to say. Mr. Potter, in the end people all do what they want to do. Sometimes people give names like 'right' to things they want to do, but how could we possibly act on anything but our own desires?"

"Well, obviously," Harry said. "I couldn't act on moral considerations if they lacked the power to move me. But that doesn't mean my wanting to hurt those Slytherins has the power to move me more than moral considerations!"

If you haven't looked at it already, you might like Eliezer's sequence on metaethics, which talks about how one can notice that our concerns are generated by our brains, and that one could design brains with different concerns, while still taking morality seriously.