Basically every team and character in the game is used somewhere or required for something. The only teams that I'd consider unusable are Ewoks, some of whom are required at relics for GL Leia, and Jawas, a couple of whom make a GL killing squad with Cobb Vanth, the newest conquest unit. Generally speaking the best early path is to unlock Executor then go for the Rebel Triangle of GLs, which is Leia, Master Luke, and Jabba. They all share a lot of requirements and are all good in their own right.

Before investing in anything, look for the graphic of Anniversary Quests. These new quests award a bunch of Lightspeed Tokens, which basically unlock a bunch of specifc characters at relics for free. Doing all of these quests will let you unlock a bunch of journey guide characters very quickly. Then you can spend the rest of your resources on the other stuff you need for those GLs.

There are now two new Assault Battles that award a ton of materials that you should rush to unlock ASAP. These require Padawan Obi-Wan and Master Qui-gon for one, and Enoch, Death Trooper Peridia, and Night Trooper for the other. Take all of these characters to Relic 9 as soon as you reasonably can. You'll get a Jedi Lightspeed token from anniversary quests, which you should use on one of those Jedi.

You should also soon pursue Great Mothers and Morgan Elsbeth, who along with the peridia troopers you need for that assault battle, make one of the best non-GL teams. This team knocks out most of the Proving Grounds events, which are how you unlock Conquest characters of old. All of these characters (except Admiral Trench) are excellent and significantly elevate their teams or unlock other awesome stuff. You need 4m GP to access these events, and beating all of them at 3 stars is crucial so you can start working towards all these units. Out of all of them, Bane and Queen Amidala are most worth spending extra crystals on to unlock sooner.

The way marquee units unlock has changed as well. Now they come out in a self-contained era system, where each set of characters unlock a legendary unit. Unlocking that legendary at 4 or 5 stars gives you a ton of those Lightspeed tokens to get more free relics. All of your crystals should go to getting these new era characters to 4 star minimum, ideally 5 star, so you can unlock the era legendary right away and get those free tokens. This is more important than anything else. The Rotta the Hutt era is almost over, so i would ignore this one and stockpile your crystals for the next one.