It could be great to introduce Sage's sons to plot as revalant characters , instead of being just legends. However it could make bad guys overpowered and it would steal some hype from Juubi or Madara/Obito which they probably will get soon, especially Juubi.
Although it could give great chance for sage son's developments (more flashback) and some insight at Sage of six paths probably
IMO it will be like that:
Edo Madara vs Edo Hashirama + Edo Tobirama
Obito vs Edo Minato + Kakashi
Juubi(remember that he has one form more) vs All Shinobi Alliance(including BM Naruto, EDO Hiruzen)
or Orochimaru suddenly betrays them and join bad guys, then it would be one fight more:
Orochimaru vs Edo Hiruzen
If it is still not enough for good guys, then Kishi will surely find way to make it more equal or even in favor of bad guys
How ?
1. Let Juubi absorb Gyuuki(Hachibi) and killing or no Killer Bee in the process, maybe even absorb Naruto and extract Kurama from him later, leting Naruto absorb some Shukaku chakra(make contact with it somehow) it would let Juubi reach it's perfect form at full power(same as against SOSP), Naruto could have some chakra from all tailed beasts, which could cause Sage of six paths soul to appear inside Naruto and talk with him, maybe even giving him all his powers/merging.
It would be awesome
or
2. As you said, just summon some Edo Tensei zombies, and it will be balanced
However I doubt any of it would be necessary, maybe first could be good. In my opinion bad guys can still keep their own against Shinobi Alliance even with Edo Hokages, Orochimaru, Sasuke etc.