i have 950 coupons by now, think that there are players with much more of them, with level increase they are more and more less of value, at least for me. I use them mainly when score event is on to purgatory resets or when need resurection book, both of this occasions are quite rare.
I would like to propose that in for example 10 : 1 ratio that there where be ability to open slots in equipment or get back gems for coupons. Collecting decent amount of them take quite a long time (for my 950 by now it took me ~4 months) so changing 100 coupons for 1 hero slot or 1 equipment slot would not be that much game balance change, and would introduce mechanism that would be lot of coupons consuming and would at least for me make coupons events much more attractive. We get new heros, new equipment pcs all along game so even opening slots for 2500 coupons would be worth 250 gold which value is no so dramatic in changing in game strenght of players.