I like playing the Science version of the game. One problem, though - with a methodical approach I've usually unlocked the whole Science tree by the time I make substantial progress beyond Kerbin's SOI. After that, the game becomes a sandbox, and beyond self-satisfaction and self-set missions, there isn't the payoff to conquering other planets. Yet to get much further without great frustration, you do rather need the majority of the science tree, so I wouldn't suggest slowing down the progress of unlocking. But... what if after the Science tree was unlocked, it was possible to go back and spend more science points to further develop individual nodes or components? That is to say, for increasing levels of science points, it could be possible to apply, say, a 5% positive increase to the stats of a part (or even one particular stat at a time - ISP, for e.g., or crash tolerance), then spend a higher level of scaled science points for a further 2.5% increase, then 1.25% etc... all the way on up to infinity, but constantly scaling up the cost and down the return, to prevent ridiculously unrealistic results. In this way, however, there would be loads more to spend your post-tree science points on, and there would always be a 'point' and a payoff to scoring more science points across the whole Kerbal system.
One or two new 'science fiction' level parts or nodes might be good, too - e.g. something akin to an 'Epstein Drive' - but priced at an absolutely colossal number of science points, to make it a real long term aspirational 'end-game' objective. Alternatively, what if you could spend science points on the 'gamble' of subsequent research that only might payoff some rare new SF discovery - so you could at least feed your late game science points into some kind of scientific 'one armed bandit'?!
