Too many coins is your problem? If only you could give some to me
Have you maxed out ALL your equipment to 115 or whatever your current lvl is?
Have you got the best possible saints on excellent saints that you could use from a DAWN to 'other hero' transfer?
Have you thought of sharing your coins with others? Power down for someone to revenge you on Monday.. they will get double rewards
With regards to runes I noticed when I got my first lvl 12 vengence that I could keep adding experience to it .. I personally don't know how it works, but why not start 'preparing' yourself for lvl 13 runes??

There are many things you can still use your coins for...