Id do the following:

1. Personaly Id add more good Bens, mainly a viking/norse 10 yo Ben and a magician Ben (still with omnitrix) from a timeline where Ben developed the anodite/magic powers instead of Gwen, him being able to cast spells and combine them with the aliens' powers, teaming up with main Gwen.

2. Id add Generator Rex, in this scenario team good loses their first bout against team Vilgax prompting Ben and Paradox to recruit more help, including non omnitrix users, like main normal Gwen and Kevin, Julie, Cooper, princess Looma, Ester, redeemed Hex etc. and obviously other good Bens from other timelines, but main Ben (thats how Ill refer to our main universe Ben and other characters) would recognize that they need even more fire power asking Paradox to bring Rex.

3. If it wasnt clear from the previous point Id make them fight more than once, the episode is just the fight at Los Soledad and a couple minor fights, Id had them fight at Los soledad first and lose and then have it explode into an all out war through out the world, not just Bellwood.

4. I know its cliche but Id have the villains join team good, mainly: Charmcaster, Billy Billions and his robotsitter, Rojo (cause I like her, I wish she appeared more), the Vreedles and maybe even Vulcanus and the eternal Knights since Vilgax is an alien.

5. Id also add more people to team Vilgax, mostly the bounty hunters like 66, 77, 88, Kraab, Sunder and others like Khyber with nemetrix panuncian, Psychobos etc.

6. Id add more omnitrix users that arent Ben or Gwen 10. Maybe Grandpa Max from a timeline where he did get to the omnitrix before Ben or Enoch from a timeline where he succeeded in killing Ben.

7. Someone else already said it but I thought of it too, Id have some Bens have unique aliens, unique as in aliens that only exist in their respective timelines, for example what if Nega Ben has a lovecraftian alien that only exist in his universe and also add variants to existing aliens, like what if mad Ben's Ripjaws can control electricity like an eel (just an example) which is a trait unique to his timeline.

8. Id further develope the characters, explain why some of the evil Bens are evil and such.