Part of me wants to say, total rewrite starting with S4. 
I'd have started the season with a storyline where Xena and Ares (temporarily mortal because
Zeus has punished him for the Dahak/Hope business) have to team up to stop Dahak once and for all, in the process learning that both Gabrielle and Hope are
alive. The problem is, if that was the case the second half of the series would be so different it's hard for me even to imagine what it would be like, and
obviously the things I like/love in the last 3 seasons would be lost along with the things I don't. So, no radical changes for me.
As a minimum, if we're talking about one thing -- I'd nix the last 3 episodes and replace them with a trilogy that would be a worthy sendoff to our heroines and would also involve Ares. I think my favorite scenario would be the one I used in my fanfic The Test, where Xena has to take command of Ares' army for a Greater Good-type cause and is then faced with the temptation of great power and of doing something that is totally against her moral code -- and makes the decision to step back from crossing that line, very nearly sacrificing her life in the process (but surviving, thank you very much). My finale would also involve Gabrielle stepping up to the plate as a warrior after Xena is badly injured, and Ares allowing Gabrielle to take charge.
In my fanfic Xena and Ares are also together at the end of the story, and quite honestly if I were to just follow my own wish list I'd do that on the show too. However, it wouldn't be very nice to all the X/G fans who had a lot invested emotionally in the show, so I don't think I would actually do it
if I had a chance to change things. Maybe I'd go with the solution of suggesting that X/A may be a couple and end it with dual ambiguity with
regard to the relationships. 
I'd have started the season with a storyline where Xena and Ares (temporarily mortal because
Zeus has punished him for the Dahak/Hope business) have to team up to stop Dahak once and for all, in the process learning that both Gabrielle and Hope are
alive. The problem is, if that was the case the second half of the series would be so different it's hard for me even to imagine what it would be like, and
obviously the things I like/love in the last 3 seasons would be lost along with the things I don't. So, no radical changes for me.
As a minimum, if we're talking about one thing -- I'd nix the last 3 episodes and replace them with a trilogy that would be a worthy sendoff to our heroines and would also involve Ares. I think my favorite scenario would be the one I used in my fanfic The Test, where Xena has to take command of Ares' army for a Greater Good-type cause and is then faced with the temptation of great power and of doing something that is totally against her moral code -- and makes the decision to step back from crossing that line, very nearly sacrificing her life in the process (but surviving, thank you very much). My finale would also involve Gabrielle stepping up to the plate as a warrior after Xena is badly injured, and Ares allowing Gabrielle to take charge.
In my fanfic Xena and Ares are also together at the end of the story, and quite honestly if I were to just follow my own wish list I'd do that on the show too.
However, it wouldn't be very nice to all the X/G fans who had a lot invested emotionally in the show, so I don't think I would actually do it
if I had a chance to change things. Maybe I'd go with the solution of suggesting that X/A may be a couple and end it with dual ambiguity with
regard to the relationships.
)
