So we have seven different probe cores available right now, and the only difference between them is their size and shape. This is useful (I definitely like the large diameter core), but the variety seems a bit pointless right now. As soon as you unlock the first probe, you've got the full functionality of Kerbal programming, then you spend another thousand science points on fitting that computer into different boxes. So, what I propose is that the different probes have different levels of functionality. The Stayputnik, for example, would be the simplest, capable only of activating stages. You could launch it using a fin-stabilized rocket, but you would be limited to suborbital (or escape, I suppose). You could still recover it for science, but it couldn't perform experiments, steer the ship, use rcs, sas, or anything else. In return, it would be extremely light, and use less power. The HECS core would be next, allowing basic maneuvering (but maybe only near Kerbin). They probes would become progressively more advanced, allowing SAS, RCS, maneuver node use, rover wheel control, and so on. The price for this would be increasing cost (science and money) and power requirements. Obviously, all of these capabilities should be available in all three sizes, I'm not suggesting that capable computers have to be big. The manned pods would not be restricted in this way. Another capability would be in 'processing power'. Basically, before any data can be transmitted, it is first processed by the onboard computers. If those computers are weak, the data volume is increased, and the maximum science value obtainable through transmission is reduced. This part would also apply to the manned pods. The one-man pod wouldn't have much computer capacity, the three-man would be better, the crew tank would be even better, but none would be as good as the best probe. This would give some incentive to bring a probe along as a processor, though it still wouldn't be as good as sample return.