Tools and Toys
The tools range from fun toys to powerful, professional tools, and most of them are free! Many integrate with Google so you may want to create a Google account just for this project - it's up to you. Most, but not all of these tools are web-based.
This wonderful tune creates interesting chord progressions for you. I tend to choose piano, turn off drums and set the bpm to around 80.
You have to pay to download the midi, but there are ways around this, using tools that turn recorded audio into midi. Although this may introduce errors or inaccuracies, we will live with these happy accidents.
Musenet takes an "intro" and then "evolves" it with AI in different styles such as Beatles, Mozart or Bluegrass. Interestingly you can upload your own custom midi track and let it evolve that too.
I tried adding a file created by Computoser as a starting point, and I think it made it more Beatles-y but it's hard to tell. Doesn't have a download option.
Tonespace lets you play one note and intelligently augments what you’re playing with chords and arpeggios.
It connects via Midi (so to a synth or Garageband) and plays musically similar notes. Because it generates midi, you can “record” it and then use it in whatever DAW you are using.