Length & Tempo
The length of your fillers may also be whatever you want, from a few seconds to up to 10, 15, or even 20 minutes or more. You should only make sure that your songs don’t get boring (especially if they’re very long), and that all parts fit together to form a real piece of art.
Your song parts can either be similar or very different, some parts may reoccur, others may appear only once. Do whatever you want, as long as it’s really interesting and cool.
If one of your filler songs becomes very long (10 minutes or more), then you may divide it into a few tracks on your album later on. It will then be a song consisting of a few parts if you want, and this will also allow your fans to directly jump to the part(s) they like most.
Think of Pink Floyd’s Shine On Your Crazy Diamond (1975) – it’s 26’05 long, it consists of 9 parts, and there is also a 3’53 radio edit. It’s universally considered to be one of their finest compositions by the way.
Your hit and slow songs usually won’t include any major tempo changes, but if you want to render your fillers really interesting then it may be a good idea to try out something like this. Think of tempo changes and crazy breaks that will merge otherwise incompatible parts for example.
And don’t forget key changes, unconventional measures (3/4, .…), or even measure changes in your songs.
Rememeber that more intelligent and more educated people often like music that’s a bit more challenging and complex, and that’s exactly the kind of music that’s missing nowadays. There’s a demand, but there’s no offer, so this may be your chance to find your own niche.