We call this the lucky 7-up Viscosity

(These are on every level up, meaning the stat has to already be there(you can't get 5 luck and then the effect shown below))

IDEA 1:
0-4 Luck - no new effect
5-9 Luck - the stat with the highest growth that didn't proc will be rolled again
10-14 Luck - the two stats with the highest growths that didn't proc will be rolled again
15-19 Luck - the three stats with the highest growths that didn't proc will be rolled again
20-24 Luck - the four stats with the highest growths that didn't proc will be rolled again
25-29 Luck - the four stats with the highest growths that didn't proc will be rolled again The stat with the highest growth out of the 4 that were re-rolled that didn't proc will roll again(essentially 3 tries for that one stat)
30-31 Luck - the four stats with the highest growths that didn't proc will be rolled again The two stats with the highest growth out of the 4 that were re-rolled that didn't proc will roll again(essentially 3 tries for those two stats)(this is somewhat pointless; if you have 30 luck do you really need more stats :p)

IDEA 2:
0-4 Luck -  no new effect
5-9 Luck - 10% chance to double a random growth upon level up for only that level up(like 85% HP becomes 170% for that level up, giving you a 70% chance for +2 HP and 100% chance for +1 HP)
10-14 Luck - 20% chance to double a random growth upon level up for only that level up(like 85% HP becomes 170% for that level up, giving you a 70% chance for +2 HP and 100% chance for +1 HP)
15-19 Luck - 30% chance to double a random growth upon level up for only that level up(like 85% HP becomes 170% for that level up, giving you a 70% chance for +2 HP and 100% chance for +1 HP)
20-24 Luck - 40% chance to double a random growth upon level up for only that level up(like 85% HP becomes 170% for that level up, giving you a 70% chance for +2 HP and 100% chance for +1 HP)
25-29 Luck - 50% chance to double a random growth upon level up for only that level up(like 85% HP becomes 170% for that level up, giving you a 70% chance for +2 HP and 100% chance for +1 HP)
30-31 Luck - 60% chance to double a random growth upon level up for only that level up(like 85% HP becomes 170% for that level up, giving you a 70% chance for +2 HP and 100% chance for +1 HP)