Awh, I love Spred... (And Spillyria, and Fresley, and Wilah, and Frangel...)
I think it could've worked out. In S5, Fred was a strong, confident young woman. She definitely was tough enough to keep Spike under control, especially mentally. She was a sweetheart, but she was no longer a push-over. She could hold her own, and Spike would've been really dear to her.
But what I don't like about it is it would've made both of them too sweet, which after a while would've gotten boring to watch. Plus, Fred was completely in love with Wes.
But, say that Fred didn't die. When Wes DID die in S5, in S6 she could've gone on some crazy out of control rampage with an uncanny similarity to Spuffy S6. Now THAT, my friends, would've been fun. 😀 Not to mention, the year after that they could really fall in love, then Fred will die for him... (Spike dies for his lovers too much) 😀