Well, considering that TIM resurrected Shep from less than a union with Morinth would have left behind, probably not a permanent end (though you can certainly end your playthrough that way if you'd like).
Yes, generally speaking, they could have given the main choices very different endings, but what specifically did you have in mind? It would have been very ambitious indeed, though. Hell, look how they wussed out on the whole issue of Tali's appearance. But a better plotted out ending (in addition to seeing the war assets you acquire--outside of the initial battle approaching Earth--actually used in combat) definitely would have been more intriguing. I'd have gladly waited another year (as would most fans, I think) for them to have "gotten it right".