I'm sure it will come as no surprise to any of you that I love epic level play. I adore the stupidly over the top adventures that heroes of high power encounter, and love the scale of such games. Therefore, I was quite pleased to see, built into the core game of 5e, a mechanism for having those kinds of games, without things getting insanely out of control for the GM.
Since first seeing the Epic Boons (DMG, p. 231-232), I have been thinking about the kinds of things I would want to see. I always felt that epic powers should break the core rules of the game in cool ways, and am really pleased with the ones presented there. However, I have a few more to add (and I suspect, many, many more to come), which I recognise may have the potential to be unbalanced...but which I suspect won't actually be that terrible, and will definitely be amazing for players. See what you think!
BOON OF DESTRUCTION
When you hit with an attack, inflict additional damage equal to 20 + the number of epic boons your possess. Once you have used this power, you cannot use it until you have had a long rest. You may take this power more than once. Each time you do, you may use it one more time per long rest.
BOON OF ENHANCED ATTUNEMENT
You can now have up to four items attuned at once
BOON OF FOCUSED MAGIC
You can concentrate on two spells or effects at once
BOON OF GREATER CASTING
You gain a virtual spell slot 1 level higher than your highest spell slot. This can be used only for the purpose of casting a spell at a higher level.