I really have wanted an Old Republic series for a while so here's what I would want in it I would there to be three factions the Sith, Jedi, and Mandalorians. The motivations of each are listed below
Jedi: Keep the peace
Sith: Control the Galaxy
Mandalorians: Destroy all force sensitive groups as they cause wars and have too much power.
The Mandalorians could be replaced with any other faction that shares those beliefs.
Below is what each season should focus on
Season 1 The Republic fighting the Mandalorians with a few Jedi going to the unknown regions to see if the rumors of the Sith returning are true.
Season 2 The discovery of the Sith Empire and war between them and the Jedi. The Mandalorian weapon that can destroy a city and the problem with the Republic fighting two wars.
Season 3 The Republic withdrawal from fighting the Sith because of the need for the army to stop the Mandalorians. The destruction of the Mandalorian weapon and swift counterattacks slowly defeating them.
Season 4 The Mandalorians are defeated and the Sith taking advantage of this by launching an attack on Coruscant causing all Jedi (Who reside on a different planet) to have to begin finding people to help with the fight against the Sith.
Season 5 The Jedi with their new soldiers fight the Sith who are weakened due to infighting and manage to take back Coruscant with the surviving Senators voting for the Jedi to be station on Coruscant so they can defend any future attacks. The Jedi construct their temple on top of a former Sith temple.
This is just my idea of an Old Republic show I think it would be a good replacement for the Clone Wars and the ending could show Darth Bane. But this is just my idea of a show.
I understand the Jedi are fighting and aren't really fulfilling their roles as peacekeepers.
I don't really have a reason for the Republic not making a new army besides the fact that their relationship with the Jedi is strengthened so they believe the Jedi act as that army.