Rewards would need to be in keeping with the theme and the lore of the game. Trophies ought to be a thing for sure, or some form of proof you were there and did the deed. Maybe even an item of another sort you could keep, something personal from each. A gun from the stormbot (deactivated), a claw from the Grim Chaser (to mount on your wall - a trophy effectively). Anything cosmetic would be great, something for your apartment. One of this games biggest USPs was/is (besides OW combat) the apartments. These need to be heralded and celebrated by all players because everyone can own them and decorate them. Its your little corner of the world and one of the major things which draws us all into the game. Lots of games have these now and so people will reasonably expect something like this to be available i am sure. It is something the whole community can share in, old or new.

If you want to give other rewards (armour etc) then make the armour resistant to the primary damage type that the mob dishes out but make it no better/or worse than the top armour in each bracket. Just give it as an alternative. Collectors can still collect this stuff but the people who want to pvp wont be so disheartened if they wore it and lost it.

Perhaps even a title could be awarded to the people who claim a kill on every one of the named bosses? Like the "paladin" myth that kicked around at the start of NC2 - for anyone who doesnt know there was a rumour/plan to give people the "paladin" tag instead of "master" on completion of all of the epics available. However it took a while for the anti-city epics to be implemented and so I think it was scrapped or forgotten about.

You could also give people a whole load of XP for completing a quest together as a faction/clan/group against these bosses. Something to bind players together in a shared experience that doesnt involve (necessarily) killing each other.

None of the above ideas are new, unique or even remotely original and every single one has been done in one game or another since the birth of the internet. You only need play a few newer mmos to get some inspiration. If something is simple to do and easy(with a pinch of salt) and most importantly doesnt impact on pvp balance then i would try my best to throw it all in wholesale if you can.

What about adding all of the other missions people sent in a few years back? Sure the competition has long finished and the winner was announced but couldnt you implement the other submissions that didnt win? Or hold a new one?

Failing all of that, just do what 90% of the other game designers are doing this year and create a battle royale mode.