Hey y'all, I've been wanting to DM for my friends, but am having trouble deciding what setting to do. I'm currently deciding between Eberron, Ravnica, and Theros. I have a small bit of plotline ideas for each one, but I can't decide which settings to use. One of my players loves sci-fi (Eberron), one loves Greek mythology (Theros), two of them don't really care, and I love the entirety of the Ravnica setting. I'm hoping for some input from y'all. Thanks in advance!

What's stopping you from having elements of all of them in the campaign? Even if you don't want to homebrew an entire setting, you can still give them microcosms to visit that incorporate the others. 

One session might have a dream sequence that's heavily Theros inspired. A city the party travels to might be magically linked with Ravnica and feature its culture and politics. An interplanar emissary from Eberron could teleport in one day and give the party a side quest. Just because you choose one setting doesn't mean you can't add anything else you like to it. 

I think of eberron as more steampunk than sifi. I’m not saying that to nitpick, just folks expecting spaceships and laser guns may be disappointed when instead they get airships and pistols. 

But no matter what you do, this is a conversation to have with your group. They’re the ones who will need to buy in to whatever you end up playing. It seems like maybe you started the conversation, so you know about preferences, but you need to finish it with them and have everyone agree on one thing. Otherwise you can end up with some frustrated and annoyed people. 

I don’t know ravnica but given your enthusiasm for it I would consider it a good base for the campaign. Then include multiple possible side quests to Eberron, Theros and maybe even spelljaming. Ideally there are multiple possible paths through all the worlds to get to the final resolution. That way they have options all the way through and are not railroaded.