mainly maths and physics. individual sites will also help
There are no good sites for this. It is simply not how science works.
Moreover, any open site that allow you to freely discuss your ideas will inevitable attract lots and lots of crackpots.
The reason why PF works so well and has quite a few professional physicists posting on a regular basis is precisely because members are NOT allowed to discuss their own idea/theories.
I think you need to know the difference between "personal" and "scientific" ideas. Personal ideas is something one comes up with without going through any theoretical or experimental test, and hence have not acquire agreement from related experts. On the other hand, an idea is deemed scientific if it at least has gone through consistency test to compare its implication with the other theories or experimental result and it should agree with them.
Does PF have a sticky post to explain what and who exactly a crackpot is in scientific debates ? General readers may not be able to really understand it except its narrow English meaning.
Scientific ideas are mainly communicated to a wider audience by writing scientific papers which get published in peer-reviewed journals, or by presenting talks and posters at conferences which are attended by peer scientists.
You may have a look at arXiv.org (pronounced "archive" because the "X" is supposed to be the greek letter "Chi") to see examples of how such papers look like.
If someone thinks that (s)he has a worthwhile idea, (s)he needs to check
a) if the idea is compatible with established theory and experiments (like blue_leaf77 wrote in post #3) and
b) if someone else already had the same idea.
For both these things, you need to be able to understand the current state of affairs by reading textbooks and papers. If you can't do this already, you cannot attract scientific interest because it is very likely that your idea is in conflict with established science or too general to be considered scientific.
Instead of trying to come up with something new, you should aim to understand. New ideas mostly come from trying to progress what's already there or from identifying problems with it.
If you read the Terms and Rules section of the forum (in the INFO tab, also a required reading when signing up), you'll find ample information on what to avoid when using PF, including what is considered personal speculation/theory (i.e. crackpottery).
in the General Content Guidelines section;
Also in the Help/How-to section:
There's also ZapperZ's Insights article on the topic:
All of the above pertain directly to rules of PF, but together paint a very good picture of what is not a valid scientific discussion.
Finally, John Baez's tongue-in-cheek 'Crackpot index' can be found here:
It's funny, but also very accurately descriptive of typical crackpottery.
BTW, if you have to ask, there's a very good chance that you do not have a "scientific idea".
Separate names with a comma.