All of those options can be handled under the current system easily enough, if the designer codes the sheet. At least the ones it makes sense to - You absolutely should not have D&D and Pathfinder characters in the same game, or 5E plus OSR characters. But one of the World of Darkness sheets copes for vampires, werewolves, mages, etc., in the same game, and many sheets have supplemental sheets for npcs, monsters, vehicles, or whatever. These can be handled by the sheet designer, when it makes sense to do so. Adding some system for GMs to add custom sheets to an existing sheet to match their needs would be nice, but the roll20 technology doesnt allow it. Adding stuff to an existing sheet requires programming, and it cant be done in a live game- you have to get at the coding interface, and write html, css, and maybe javascript. The best sheet designers could do is add a Custom Tab, and add some text boxes and repeating sections, for GMs to name and use however they want. But it's impossible to really plan for what GMs would need in advance, that such an approach owuld be pretty rudimentary. In my homebrew sheets I usually have such a tab for GMs to note down common rolls they need when they dont want to make a character for them, and notes they take during a session, but there's no standardised approach to this that would suit everyone. On a related topic, if you want to convert characters and creatures between different game systems , the character sheet system will never be able to do that for you. You have to decide what you want to keep from the original system and how you convert the stats. Every GM will do this differently. The best way to do this is what I suggested above: open a copy in a different browser, and decide what stats you want to transfer over, and what you want to change them to.
