Currently, the highest level of monsters in story Acts 1-III (IV) is level 70 in legendary. I propose that just as you are able to adjust the level of monsters after completing normal mode, so, after completing TL 71, you should be able to adjust the level of monsters in the story levels to equal that of the highest trial level you have completed (or to equal that of TL 100). This would have two benefits: first, it would enable you to "practice" in story mode against higher trial level type monsters, and second, it would allow you (if you wanted) to, for example, combine EXP and Gemstone farming more efficiently than is currently possible by running TBD set to TL 80 or 90 or whatever difficulty. There would still be a tradeoff between EXP gained and Gemstones acquired in this scenario--the higher the EXP, the longer to kill the monsters, the fewer overall runs you can make, the fewer the gemstones collected--but it would be overall more efficient and fun than the current set up, where you have to run trial after trial for EXP and nothing but that, and in which you have to run TBD (I know SoA is better mathematically but it's SO BORING if you run it the right way) for gemstones for little EXP gained.
This would benefit all players above TL 70 and would disadvantage no players below TL 70.
This would benefit all players above TL 70 and would disadvantage no players below TL 70.