pp:
>>ball bearings might be more difficult to find a version that can be used for many models<<
Thats why I suggested one above.
Here's how I came to the conclusion that this would be a good one:
Up to a certain point smaller is better because: for bigger parts you probably need bigger bearings only if you want to run them under high speeds & loads, but thats not the case in the majority of potential prints here I guess.
So I've choosen a bearing with an inner diameter just enough that an fitting axis would be printed considerably round
(3mm -> 30layers). And with an outer diameter (6mm) which still can be integrated into smaller designs.
Also it was the smallest bearing to which I found a "Freilauflager" (=sliding bearing?) with same inner diameter (this: HF0306, 3x6x5).
The smallest ball bearing I found was this teeny weeny (681 offen, 1x3x1)
with only 3mm outer diameter. This is not so far away from the 3mm one.
So I think 1, 2 and 3mm inner diameter bearings are the most interesting.
There is the same problem with the steel balls which sizes to choose.
I think the problem with bearings is rather that they might not be adepted
because ball bearings are only usable in combinatory manufacturing
and that is not exactly what everyone does.
(But maybe it would enforce that?)
About magnets:
Personally I haven't desired magentes yet but, i'm sure others will.
Here again: which sizee, which strengths, which geometries?