Honestly, part of it has to be the granularity of the system. Healing Rate is currently CON/6, round up, because everyone wants to use whole numbers. If, instead, you chose not to round up, you'd have something a little more interesting, if a bit more complex. Your CON 13 character would have an HR of 2.167, while CON 18 gets you a full 3.000. Requires the tracking of fractional hit points (or a revamp of the entire hit point system) but would give you the granularity you want.
Gets harder with Luck Points, where they replenish back to their starting value each session. You'd have to tweak the rule, to say that you get back your base value in LP, but do not lose any fractional LP that you have left in your pool - so that 2.167 LP guy ends the session and then gets points back to take him to 2.33, and eventually, after six sessions, he starts with 3.00 LPs and has an extra whole one to use. Again, you have to be willing to track fractions or revamp the rules entirely.
Experience Modifier is actually a very close cousin to the way that prime requisites worked in OD&D - you had a high enough score in your primary stat and you got a 5 or 10% bonus to earned XP. It, too, was a static breakpoint that made any stat over that value effectively useless. At least here you get some skill default bonuses for those high ability scores. I suppose you could do the math-heavy version here as well - for every point of CHA below 7, you have a 16% chance of losing an Experience Roll. For every point over 12, a 16% chance of gaining an extra one.
I'm afraid to try to tackle the fantastic breakpoint that is INT+DEX = 25 - 3 APs and benefit from rounding up on Strike Rank - ain't nobody going to sit still for rolling a percentage chance of getting an extra action point per turn, no matter how realistic it might be...