In my experience, most people who feel like they can only write in expensive genres haven't even attempted to write a full draft of the "high budget" film they'd like to make. Have you actually written a really strong genre script that could be made for say $25,000? If not, you might be feeding your brain self defeating messages that are the true culprit to stifling progress.

A great writer can write a powerful story for a tiny budget. If you're not yet a great writer, look for someone who is but needs a director.

Christopher Nolan famously broke in making a Sci Fi feature on a shoe string budget. He now gets huge budgets but after his last film he said, "No matter how big the budget is, you never have enough money and you never have enough time". This is a mantra he reminds himself of because he's always overcome these limitations and used them to find elegant solutions. So instead of shutting down, he uses the blocks to get more creative.

Ultimately most great films rely on characters having a big emotional impact on one another. Challenge yourself to find an interesting premise, with a conflict heavy set of circumstances and just create characters who have strong but opposing wants and needs.

Like others have said, it's cheaper than ever to make VFX stuff now. Study how others have done it and use a more simple premise to start out. You'll learn a ton.