My advice is, try to merge all yer ideas into one single wad. I had exactly the same "problem", as I kept brainstormin' on new ideas, new level concepts and such. Then I asked some fellow members about if this and that can be accomplished under GZDoom, and when they said "yes", I decided to carve a niche for these special levels in the project.
The best thing is having a decent storyline which you deliberately leave open for new ideas you might figure out afterwards (this eliminates the necessity of completely rewriting sections of the WAD just because you had a change of heart in the very last minutes). For instance, I always admired good space-shooter games, and I thought it'd be a nice change to create such inter-dimensional levels, where you fly openly in a seemingly borderless space with excellent animated sky givin' the illusion of flying, you have to dodge asteroids, novas, starbursts etc. while you can grab bonus items hovering in mid-air. Soon it became evident that such level in fact can be done quite easily by a couple of invisible walls embracing a wide space, using sector-wind and zéro gravity, plus a nice animated sky. And there - I can create space-shooter-like levels, which is truly a nice change after so many walking and shooting everything in sight. This is how
Asteroidz,
Ast-city and
Ast-rock will get into my TC.
Also, creating Easter Egg levels is also nice. As you might have noticed, Doom E1M1 appears in many other FPS games as an Easter Egg, let it be official or fan-created level. I've always loved hidden Easter Egg levels based on something irrelevant of Doom. So far, my two Easter Egg levels were
Goodbye Kitty!, and
Hypercubed!!!, referring to Hello Kitty and the movie Cube 2: Hypercube. I shall also build Easter Egg levels for my TC, the
Ourang Medan referring to the famous ghost ship of Indian Ocean, and
Lost in da Mist, referring to the Frank Darabont movie based on Stephen King's novel.
As for the storyline, it is unquestionably better to have a storyline which defines the environments, the situations, the happenings, and pretty much fixes your ideas about what levels should you create. IMHO story is very important, after all, it gives a meaning why do you actually play this wad rather than a two-sentence "story" telling you to "go there and kill thingz". As someone cognizantly pointed out years before, people like to see some new stuff in each project.
And what keeps motivating? The success so far, of course!
