Most games that have a progression system tied to luck have some sort of pity system. For example, card games guarantee you a Legendary after X pulls without getting one. Gacha games allow you to combine duplicate characters to make them more powerful. However in PRO, when you get a bad Reroll, nothing happens. All you did was waste your resources and all the time it took to collect those resources.
My suggestion is: when you Reroll your Legend and choose to keep the old IV set, you get to add 1 IV to any stat on that Legendary.
So, if this were my current Lando and the Reroll:
I could keep my current Landorus but change the DEF to 9.
This will alleviate some of the frustration that comes with wasted Rerolls, help players progress towards a Hidden Power they might want, and possibly tempt players to sink more resources into some of their legends that are good but not great.
For example, this is my Heatran.
I would love to have a better one but I know that keeping a high speed IV and overall improving the other stats is extremely unlikely. So, under the current Reroll System, I will probably never Reroll this Legend again.
Thanks for reading