I’ve seen a couple similar suggestions made before but I figured I might as well throw in my 2 cents on the matter.
I propose the following tweaks :
1. Make it so that delves start at lvl 10 instead of 20. ( not completely related but I find it to be an odd design choice )
2. Have the treasure multiplier scale by +0.1 per 10 lvls, starting after lvl 10. This would make the starting multiplier on lvl 100 delves be 1.9 & 4.9 for lvl 500 delves.
3. Adjust the first time completion bonus to +1.0 for lvl 10 - 100 delves, +1.25 for lvl 110 - 200 delves, +1.5 for lvl 210 - 300 delves, +1.75 for lvl 310 - 400 delves, +2.0 for lvl 410 - 500 delves.
Considering that delves are generally speaking a limited thing to grind and that as it stands atm the incentive to do higher lvl delves as opposed to lower lvl delves is almost nonexistent. ( other than for the renown of course ) I believe that changes along these lines are long overdue.