Some of my ideas. All are free to use if you find them useful.
On speed of leveling
Speed of increasing in power should be reduced compared to 3.x, definitely. It shouldn't be exponential with time, as in these editions, and it should take more both in-game and real time to reach world-shaking power levels.
On the other hand, speed of levelling should stay the same, or even increase. What I mean by this is that each level should be only a small increase in power, and rather linear than exponential, but levels should be gained often, even about once every session. This way, players feel the character developement (they don't have to wait for long to get something new), but you don't run into the 20-in-20 problem.
On 15min days
What causes the problem? Depleting the resources too quickly, loosing power because of it and being arfaid to proceed with now weakened characters. Resources may be regained by resting, so the party rests to get back to full power.
Maybe what is needed here is a change in paradigm. It won't fit a strongly simulationist style, but will definitely play good in more cinematic one. Let characters GAIN resources during ENCOUNTERS instead of rest. And I don't meen once-per-encounter powers.
Imagine a pool of points - call them as you like - that are gained at steady rate during any confrontation, maybe just one per round. Different powers and abilities the characters have are paid for with these points, cost increasing with power. Couple it with damage mechanics without depletable HP (damage saves, wound thresholds) and powers that allow players to ignore damage or keep going despite of wounds. This way, instead of fights typicall for 3.x, with biggest nukes in the first round, either bringing instant victory or followed by defeat, you will get ones beginning slowly and ending with fireworks. You'll keep resource management, but shift it from "use it now or save for the next fight" to "fire A now or risk holding on for the next two rounds and finish enemies with powerful B". While players are still planning, for characters it may be rather about increasing determination or anger, not tactical thinking - and I think it fits typical fantasy game better.
On system mastery
Use parallel, instead of interweaving, character developement options. Each skill/feat/merit/spell/whatever should give a character something unique to do, but it shouldn't be possible to stack them for increased effectiveness in a single task. This way, all choices may be meaningful, but you won't encourage (and, effectively, require) building combos and "optimizing".
On magic items
In my opinion, the best way to discourage changing the equipment with levelling is getting rid of scaling items. I know, that "+X" items have been in D&D since the beginning, but I don't think they add anything to the game. Same with stacking multiple powers on a single item. If most items rather give unique powers than buff characters in their areas of expertise, players are more likely to enjoy what they get instead of changing their equipment for better one every time money and opportunities allow.
On speed of leveling
Speed of increasing in power should be reduced compared to 3.x, definitely. It shouldn't be exponential with time, as in these editions, and it should take more both in-game and real time to reach world-shaking power levels.
On the other hand, speed of levelling should stay the same, or even increase. What I mean by this is that each level should be only a small increase in power, and rather linear than exponential, but levels should be gained often, even about once every session. This way, players feel the character developement (they don't have to wait for long to get something new), but you don't run into the 20-in-20 problem.
On 15min days
What causes the problem? Depleting the resources too quickly, loosing power because of it and being arfaid to proceed with now weakened characters. Resources may be regained by resting, so the party rests to get back to full power.
Maybe what is needed here is a change in paradigm. It won't fit a strongly simulationist style, but will definitely play good in more cinematic one. Let characters GAIN resources during ENCOUNTERS instead of rest. And I don't meen once-per-encounter powers.
Imagine a pool of points - call them as you like - that are gained at steady rate during any confrontation, maybe just one per round. Different powers and abilities the characters have are paid for with these points, cost increasing with power. Couple it with damage mechanics without depletable HP (damage saves, wound thresholds) and powers that allow players to ignore damage or keep going despite of wounds. This way, instead of fights typicall for 3.x, with biggest nukes in the first round, either bringing instant victory or followed by defeat, you will get ones beginning slowly and ending with fireworks. You'll keep resource management, but shift it from "use it now or save for the next fight" to "fire A now or risk holding on for the next two rounds and finish enemies with powerful B". While players are still planning, for characters it may be rather about increasing determination or anger, not tactical thinking - and I think it fits typical fantasy game better.
On system mastery
Use parallel, instead of interweaving, character developement options. Each skill/feat/merit/spell/whatever should give a character something unique to do, but it shouldn't be possible to stack them for increased effectiveness in a single task. This way, all choices may be meaningful, but you won't encourage (and, effectively, require) building combos and "optimizing".
On magic items
In my opinion, the best way to discourage changing the equipment with levelling is getting rid of scaling items. I know, that "+X" items have been in D&D since the beginning, but I don't think they add anything to the game. Same with stacking multiple powers on a single item. If most items rather give unique powers than buff characters in their areas of expertise, players are more likely to enjoy what they get instead of changing their equipment for better one every time money and opportunities allow.
