I have a post in general with some suggestions about difficulty. I think there are some good comments in this thread though about limitations. Larian most likely isn't going to go too crazy implementing high level options like OP suggested. We really need to come up with suggestions that are easy for them to implement and don't compromise the overall feel of the game. To that end my main suggestions are to limit xp and gold through a checkbox on tactician mode (or roll it into an insane difficulty).
Gold makes things easier - making it much rarer would limit your gear choices and make fights harder.
XP makes things easier - as we get more tools through levels, a veteran player has more at their disposal to max/min and overcome the challenges.
While there are numerous creative ideas that could be fun - this simple simple setup could really create a great standard for a higher difficulty.
