In the past I had various projects at different states of completion, usually different scales, different companies and different rules. Most would get abandoned due to the amount of work and I would look for something new and shiny that was more exciting.
After dealing with this for a long time I decided to try settle on a single scale and period, the idea being that it could be used for either small or medium skirmish games, which are my preferred sizes for games.
My big project for the last 2 and a half years or so has been 28mm Sengoku Jidai Japanese. I've completed and painted about 80 minis and a good amount of terrain and I made a fair effort not to start other projects, after many years of uncompleted projects. I'm happy with my progress but occasionally I did just need something different. So I have a box of Perry AWI Continentals, built and abandoned. Also a box of Perry Prussians, yup, built and abandoned. Some Foundry minis for Old World Warhammer skirmish ... actually primed and ready to paint. Also I bought the Blood Bowl 2020 box on a whim and I really enjoyed building the black orcs, which are primed and ready for painting.
What these diversions made me realise is that I will be sticking to Japanese for my medium model count gaming. I really couldn't start any other period with 50+ minis a side, it's just too much work and expense. But blood bowl teams, small skirmish bands for various games make a nice distraction and change, so when I go back to my Japanese I have a bit more fire in my belly than if I just plodded on with those alone
Edit: although for a long time I've been toying with the idea of doing some skirmish in 6mm or 10mm, on a shrunk down table. Don't know if it would work for something like Pulp Alley but maybe larger skirmish. One for the future perhaps