This one crossed my mind after discovering the upgrade-able buildings update. I was thinking of certain Kerbals have certain skills that improve over time as they fly more often until they reach a fixed point in that case they would have 'mastered' their skill. For example a Kerbal that can decrease fuel consumption by 1% at first, it wouldn't be very much but after he does his first flight it rises to 1.5% (It would probably be best it will be harder to rise that over time) Over a while of repeated flight attempts his skill rises up to a maximum of 5% which would affect Delta V by just a bit but that would just be the result of 1 Kerbal, having 3 in your pod will add quite a number of bonuses and it really gives more 'loss' as you lose your Kerbal in a crash. If you had the Mk1-2 command pod with 3 Kerbals applying 3 different bonuses, especially if they are skill masters. Your rocket would be more effective So far I have a few Bonuses as an Example 1-5% Bonus to Thrust from Liquid Fuel Engines/Solid Fuel Boosters/Jet Engines 5-25% Reduction to Air Intake requirement for Jet Engines 5%-25% Increased Electricity from Solar Panels 10%-30% Increase to Electricity Capacity These I believe may make an impact to KSP's Career, Sandbox and Science mode for gameplay
