I always hit this point in development when...

[MrBehemoth]

I was talking to my other half about this kind of thing recently. I think the problem is that making games, whether you're modding or doing it from scratch, is a craft involving very long term projects. Painters paint a painting and it's done. Musicians create a piece and then they can get on with performing or distributing it. Game developers (and writers and filmmakers) can spend months making adjustments just to see a tiny bit of progress. There's a danger it might start to feel like work! Nooooooo! We're creative, imaginative people, and we go through a cycle of imagine > create, imagine > create, etc..While we're stuck on the creation part, the imagination gets bored and wants to imagine something new.

Call it Delayed Imagination Fatigue. I think it's something that happens more to people on solo projects, because other people's input can be unexpected and give you a fresh perspective. Two things I do to help keep the imagination happy while it waits for the creative process to catch up:

1) Have two very different projects on the go, and spend a week or a month on one or the other. Make them stylistically or thematically different. Maybe learn how to mod a different engine, so that you've got some vastly different assets to play with. (But make sure you come back!)

2) Break your project into modules, a bit like what AGP said, but I like to do it on a larger scale. Do have a overview of the whole project, but don't plan out every aspect of your game/story. Just work on one module at a time. If you get bored, move onto a different module for a while, but resist the temptation to plan it out completely. I like to break the a story into chunks. I'll create the beginning and sometimes the end to give myself a target to work towards, but I don't know exactly what happens in the middle because I haven't planned it fully. Then, there's a motivation to work towards the day when you get to find out what happens, you reveal the missing pieces of the puzzle to yourself. Create the narrative in a way that lets you experience it yourself as you go along, otherwise your imagination will want to move on to something else long before you finish creating.

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Edit: 3) Oh, and play games. Lot's of different genres and styles, not just the genre and style that you like to create. Play them analytically and learn from them. Ask yourself, "what can my Amnesia CS learn from Mario Kart?" You'll come back to your mod with some fresh ideas to get excited about.