Can I create a special potion or enchanted item with a mod?

Use Tynker's mods editor to "change the rules." Create a new mod by going to Projects > Create New Project > Minecraft > Create Mod

Try out a short program like this one:

Or use a block like this one to give the player an effect:

Explore Tynker's Player Effects category for more coding ideas (Project > Create Project > Minecraft > Player Effects)