The most effective way to get rid of the 15 minute day would be to give characters abilities that ramp up during combat and decline at rest...
Say that every combat gave your fighter a cumulative +1 attack bonus on future combats for the day... and your warlock got a cumulative +1 damage bonus on the eldritch blast... those characters would be encouraged to rampage around and get as much done as possible.
Now if you balanced it and gave the fighter and warlock each one or two "once per day" abilities... then they would have to balance between pressing onward for the increasingly greater bonuses... meanwhile they'd have to preserve their 1/day ability for when they really need it.
Say that every combat gave your fighter a cumulative +1 attack bonus on future combats for the day... and your warlock got a cumulative +1 damage bonus on the eldritch blast... those characters would be encouraged to rampage around and get as much done as possible.
Now if you balanced it and gave the fighter and warlock each one or two "once per day" abilities... then they would have to balance between pressing onward for the increasingly greater bonuses... meanwhile they'd have to preserve their 1/day ability for when they really need it.