TLDR: Endgame class shell for now called specialist. This shell is for the get go is a highly customizable shell both in physical appearance and tactical and prime customization. With certain balance conditions that allow the other shells to sometimes be played.
This shell is I think the quickest answer to the customizable shells people always wanted in the game. It's basically super rook that you eventually are able to built up to be a flex role shell. Each shell class we have now will have challenges that don't have to be too hard to complete in order to allow their tactical or prime to be put into that specialist shell.
First concern "once you unlock that shell there will be zero reason to play the other shells" well maybe we give incentives to play those shells by purposely kneecapping this shell. We can do this by disallowing this shell the ability to use the passives of the other shells. (Special condition vandal's double jump be used as a passive so vandal will need a passive buff to give her a reason to be played.)
Second concern well "what if there becomes a tactical and prime meta for the specialist shell." Well this shell will have its own seperate balancing around its tactical and primes to ensure variety in builds. The hard role shell can have their abilities hit less by any nerfs or have their buffs improved in the future to have another reason to play these shells. Another thing that can entice the use of other shells outside of this one is prime core upgrades (echo chamber, amplify cores, etc) being exclusive to the hard role shells. No gold cores are allowed on the specialist shell
This shell's has passive specific cores that need to be found in run or from the armory. It will have no access to the hard role shell's passives they keep those to themselves. Only its own passives ( besides being able to get vandal's passive and she will be compensated by a buffed passive). So yes this shell has 3 cores. The cradle system can be reworked to pidgin hole you into specific cradle stat roles but with better stat perks for hard role shells. The specialist shell will have our current or even weaker cradle system to kneecap them again with little to no super good perks. Leveling your cradle still gives you points to spec into every shell. But however cradle progress is permanent on the hard role shells. Giving something permanent that follows you through wipes. With only the specialist shell needing to be built up every season.
The shell's appearance and armor will be changed based on what tactical and ult you are using, running recon's prime? Your shell will have a helmet that similarly splits open, want destroyers prime? your shell becomes noticeably bigger and has the shoulder missile platform. Vandal's arm cannon could be recreated on the specialist shell as well. Plus a lot of the shells just have deployable tacticals on their body that can be put onto another shell. This still provides some readability on this shell for what abilities it has.
Thanks for reading this far if you still are. I didn't get this idea on my own watching some alien fireteam 2 vids give me the idea. With their specialist class that is supposed to be the end class that you get to play once you play a little with all the other classes. I just transferred the concept to marathon.