You've done it. You've slain the dreaded lord of the firestorm citadel, bested the snarbolax, the vicious royal jelly, and the near indestructable roarmulus twins. And for what? A fancy symbol next to your name and a pat on the back?
This has most likely been brought up many times before, but I believe obtaining the rank of vanguard should be more rewarding, for example:
You know all those trinkets they gave you as rewards for completing the various missions? You can't benefit from having them without a trinket slot; which is why I think when you become a vanguard, you should receive clearance to a permanent, soulbound trinket slot. It wouldn't subtract much from the income of OOO rings, as there's a second trinket slot to be opened and 2 weapon slots, alongside people being able to buy them off the supply depot. Alongside all of this, it would require hardly any dev time. If all goes well and this is added, more people might get into lockdown as it's hard to play against people with double elite quick strike gauntlets, snarbolax gear, and a gran faust (AKA the generic lockdown player), where at least you can endure them if you have a heart pendant.
There are plenty of other options to make vanguard rank more rewarding, this is just my opinion.
I don't think a permanent trinket slot would be the best idea, especially since our battle sprites already fulfill that role in the clockworks, however I do think you're looking the right way.
Reaching vanguard rank really doesn't have the be the trigger. At vanguard rank you will gain enough prestige for the first prestige rank. Thus, this sort of thing could be linked to prestige, therefore any player could technically use it with a high enough time investment.
I would say that every rank of prestige opens up a "prestige slot" on your player. These could be in the costume tab. What these slots do is provide any tier 1 perk of a battle sprite. This means you can have bonus damage vs certain enemies of low, status resists of low, damage resists of low, and also +1 HP. Since there are five of these medals, you could thus have +5 HP, +5 status resist, +5 to normal resistance, etc.
Since prestige cannot be earned through lots of money (except for arms appropriation), it makes the game less pay to win, and more play to win. It also means all vanguards essentially have a 0* or 1* trinket always.
Now how to potentially balance this?
I can already see some unbalanced stuff happening, so let's get it figured out quickly.
+5 HP might not seem that bad, but for fully geared players, that could make a striker have more health than a guardian in tier 3.
For each strata, a perk is enabled. In stratum 1, all prestige perks are disabled. At stratum 2, the first slot unlocks. At stratum 3, the second one unlocks, etc. Since all the unlocks are technically 1*, this system would have to do as a scaling one simply wouldn't work.
Now with this in mind, this balances all potential complaints about PvP, since all PvP games take place at certain depths. Tier 1 PvP you only can have 1 slot, tier 2 you can have 3, tier 3 you have all 5. But this still makes some unbalance happen in the still unbalanced tier 3 PvP, sooo...
+ Add criteria to certain slots. Potentially in loadouts marked for PvP (or just in general), you cannot have more than 2 of each type of upgrade. That means you can have +2 HP, +2 shock, +1 curse. Alternatively you could even prevent more than 2 of the same type of thing, such as not allowing shock and curse(because both are statuses), but instead you can choose to have +2 to a status and then finally have to chose what damage type to slightly resist. This allows for a lot of specialization, but you can't just specialize in one area.
Tying a reward into prestige instead of vanguard seems a lot more rewarding as it keeps players around longer... Plus if you've never done a prestige mission you'll still unlock your first slot for just becoming vanguard!