I am currently working on a homebrew subclass that I plan to make modify how many sorcery points you have. I have not found anywhere that will allow you to add more. Since this is a homebrew thing, it can easily be tracked to the side off of the DnD beyond character sheet. But I think this could add for some nice extra options when creating homebrew if you could grant bonus sorcery points through abilities, or at least the ability to raise the cap higher manually in the character sheet. Thanks!

For now, the only homebrew modifiers are things that official subclasses and feats need. Any custom modifiers probably won't happen until custom classes, which will be a while.

You can always just make a feature called "extra sorcery points" with limited uses to work around it. But there is no modifier for the base class feature (in fact I'm pretty sure regular sorcery points work exactly how I described).