allowing you to enchant things with no boundaries, i.e. largest range/duration/effect as you want with any soul gem. Or one like it?

Search Supreme Magicka on TESnexus it increases the enchanting freedom among other very usefull features. But the enchantments are still capped.
If you had something specific in mind it will usually be easier to just make yourself the item in Construction Set and place it in a container somewhere.
Also there is another mod called Enchanting Freedom, google that and you should find it. It adds effects that would normally be locked such as spell absorbtion and reflect spells.