This "problem" will disappear after the first Era.
In 3 months, you will have 7 new characters to gear, mod, and figure out their place in the meta while the next set of Era units is in the Colonoscopy--pardon, Coliseum. It will probably take you 3 months to finish shard farms, gear, mods, zetas/omicrons etc, so by the time you're done with that, the next Era's units will be joining the legacy roster.
And while we're waiting for Stormtrooper Luke and friends, we're gonna get Smilo Ren. And we have the new relic economy to play with and increase relic levels on our existing squads while we're waiting for Luke and the others. It's not like there's nothing to do in the game for 3 months.
Unless your entire roster is R9 with perfect mods, you don't have nothing to do.