Thinking about my most recent character here are some relatively small changes that would make playing more fun. They are all based on eliminating tedium-based or lame min/maxing mechanics, and most have been mentioned before. If you think a game should reward easy but boring work then don't bother reading further.

Eliminate skill practice: Instead give bigger level-based increases and add time-based increases. E.g. every level and every 5 play hours you get a (wis/int based) increase to all skills. Reason: Right now an optimized character trains all skills once and then just practices from there. It's a direct trade-off of tedium for power.

Base all level gains on max race stats. No more permanent gimping due to forgetting to swap on gear, or dieing a bunch. Reason: Will encourage pre-hero pk and exploration. Also having to remember to swap gear before leveling is just tiresome. Right now current-stat-based level gains don't really balance anything, they're just a mindless hassle, and only at low levels.

Age-based stat changes don't waste trains. E.g. if you max your strength while young, and it goes up at mature, and then down at middle-aged, you won't have wasted a train. Reason: It's silly to make people plan out trains like this. More importantly if you don't know about it you get screwed for no reason. At least it's in the helpfile now.

Give a choice between exp penalties and triple-con-loss against non-cabal mobs. Exp penalties aren't like they used to be (My personal best was a 300k deficit) but I'll pay pretty much any price to have to rank less. This change only makes sense with the max-stat level gains change.