Anyone that knows what I'm up to will know I'm all for 3D printing, custom parts and the game to be accessible to all.
However I side a little with CitrusNinja and CheetoBlader in the sense it's a good idea, it keeps coming up and nearly every time it's some ideas in a new thread rather than something quite solid, it also I think gets seen more complicated than it should be.
I do also each time avoid on posting my thoughts / knowledge on the subject as I previously have been involved in rulesets for RC car racing as well as finding my own loopholes around them.
1) Materials to be restricted and the manufacturing methods.
This is both on a cost and safety basis. £200 filament printer technology I wouldn't put on this list, sorry.
2) Maximum diameter and the circularity of the design restricted.
The earlier Metal Saga succeeded in this well by having no designs over 46mm from memory, it naturally stopped them being too dangerous or powerful due to the size. Also nobody wants to see complete circles, they're just boring but is tricky to word. Best I have come up with is no more than 75% may have the same continuous diameter by design.
3) Maximum weight restricted.
Self explanatory, again Metal Saga succeeded well here not going over ~45grams from memory. They stay quite safe at this weight but gives good design freedom.
4) Modification of parts from their original design.
Kind of pointless allowing own designs then banning further modifying them, unless it is blatantly dangerous.
5) Sharpness of contact points.
I think this regulates itself if you have enough sense to be designing something you want to win, something sharp will likely have huge recoil so will be pointless to design it. Plastic Gen and Metal Saga I think generally had 90degree contact points with no radius and both performs well, looks nice and is relatively safe.
6) Mechanical system to use.
Should it be open or restricted is a big debate. HMS and Metal Saga are in my opinion the most suitable for custom parts, Burst is suitable too but Metal Saga is the easiest to understand and make parts for. It restricts complicated designs.
7) Cost of the parts used.
Materials choice limits this and limiting the cost makes it more accessible, but one offs aren't cheap. £75 / $100 is a realistic limit for a one off, but isn't accessible to all.
8) Parts available for others to use.
Be it the parts can be sold to other players or the files are shared, it stops arguments when something dominant comes along and isn't easy to copy. Goes against the Anime philosophy in a way but makes the experience generally more friendly.
9) Wear gloves.
You should do really with anything that contains metal contact points that spin.
10) Keep it simple.
As stated by others, most don't bothering reading rules anyway. Those that do, often are looking for a loophole in them and the fun gets lost.
I hope that gives some insight from the experience of having had many custom Beyblade parts made for all 4 eras over the last 8 years. The above is the best and simplest I've ever come up with and would advise the Metal Saga is the best base for a custom design.