Adding things to the game that have an actual RXP cost is interesting honestly. It has a lot of applications that mostly come into play at 156+, but you /could/ do it earlier if you made that decision though I presume few would unless the reward/cost ratio was very in favour of the player - and knowing SSG that would not happen should such a thing come to be.
You could have RXP costing consumables - or ones that only work in Reaper. You could have cosmetics, etc. They could work as an alternative carrot post 156, or people could use their rxp to buy stuff if they have no intention of ever running stuff above R1 and mostly prefer heroic/epic tier, where the perks of having excessive amounts of rxp is negligible or downright unfun for some people.
In general, there seem to be a few things in the game that most people have in excess (plat) and some people have in excess (rxp) that we have no sinks for - I think there need to be more sinks. At least platinum sinks, but rxp could be another thing we could throw a sink at.