Adding new characters is great and all, but it can be overwhelming as a new player to only have 5 or so characters and see how slow getting new characters is. I myself am level 61 now with 35 characters, and even I still feel like it will take forever to get the 41 remaining characters. While this is technically good, as it means there is plenty to do, not everyone sees potential when they look at a blank slate.
I suggest allowing multiples of the basic characters. NOT all of the characters, just the ones that are not specifically named. Imagine having 5 Count Dookus... that would be ridiculous. So for example, there should only ever be 1 Qui-Gon Gin, or 1 Eeth Koth, etc... But why can't we have 2, 3, or even 4 Jedi Guardians? Or when we eventually are given access to the battle droids, why not allow us to be able to have several instead of just 1? Thus, in order to make this work, there would need to be a rarity system. I suggest having a 4-tier leveling system.
Rarity level 1 = 1 each (named characters/ really powerful)
Rarity level 2 = 2 each
Rarity level 3 = 3 each
Rarity level 4 = 4 each (very week, some shards available in normal light/dark side battles)
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Why do we need so many extras? Because there is more to the game than competitive player vs player. Just looking at players around level 40, there are the regular light/dark battles both normal and hard, cantina battles, challenges, events, and the Galactic War. Having access to many extra (albeit weeker) characters would be a huge boon especially to starting players, but also mid and high leveled players in all of these.
Looking at the clones, yes, the three of them make a nice set, but how many players will actually have all three of them before level 60 (even then)? Fives can be bought at the cantina, or farmed in Dark2-DHard and Light4-FHard, but Rex is only available in the data packs. Those who are willing/able to fork over tons of cash could get Rex early on, but non-paying players are left out unless they are extremely lucky with their draws.
By giving access to extra basic clones, they will allow the bonuses to be used to their full(ish) potential for new players without making them super overpowered (because they are noticeably weaker than the higher rarity level characters). But they can still be a benefit for veteran players (such as being thrown in as cannon fodder in GW). Plus there are countless other ways they could be used in future content.
Now, some characters such as the Tusken Raider actually become pretty nice once fully upgraded, and if there were say 3 of them, then they would become a little OP. To counter this, there could be something like the "Veteran Tusken Raider (lvl 1)" and then just "Tusken Raiders (lvl 4)", The Veteran would be what the Tusken Raider is now, while the non-Veteran would be a very dwarfed version, just something to make new players salivate over when they get the second or third and make them really want the better ones.
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The way getting a duplicate character would work, is when you have enough shards to upgrade it, you can instead chose to make a new character. All equipment and leveling and future upgrading will affect all like characters. The drawback though, is that the shard increase is unaffected. Promoting a Clone Sergeant from 3 to 4 stars takes 30 shards, and from 4 to 5 stars takes 65 shards. To make a second 3 star Clone Sergeant, one would pay the normal 30 shards as though upgrading. Then upgrading both of them to 4 stars would cost 65 shards.
The Clone Sergeant would be harder for a new player to take advantage of having multiples of. But I suggest that the super week characters, such as Clone Trooper or B1 Battle Droids be available to farm from normal level battles. These characters would be extremely week compared to other characters, but since they can potentially be upgraded more quickly, they would give new players enough of a boost to feel as though they are getting deeper into the game more quickly, and be able to fight somewhat competitively by matching the weaker characters with better ones that have abilities that benefit from having a lot of a specific type of character.
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Even though people would be almost guaranteed to last longer in the Galactic War (though with more losses at a time with all the weaker units) the rewards would probably not need to be adjusted, due to the fact that there will be so many more things to use the rewards for. Plus the weaker lower rarity leveled units would eventually become strictly either ability buffing units, or part of suicide teams. This would open a great opportunity to have extra Galactic War settings, such as limited to Light/Dark side characters (fight against the opposite), or having more battles before finishing it (a harder difficulty after obtaining your 30th character, or your 10th 4 star character, or something).
Then there are so many other options available for this. For example, new challenges which are limited to other specific types, such as permanent versions of the Droids vs Jawas (or at least making them reoccurring events).
Players who just like having a super strong team may not care about this, but the strategists who enjoy having a main team and then developing secondary teams would get a real kick out of this.
Also, check out my thread "New Content Idea: SURVIVAL!" to see another way they can be used really well.