I would like to see alternative ways to play the classes, like replacing Iron Halo with Fragstorm Launchers, or a unique melee weapon for Tactical of just the bare fists of the marine. So we can live out our punching fantasies of burrowing through a horde of enemies of The God Emperor with our Faith and Fists. I know it wouldn't be as effective as other melee weapons but it would be pleasing to just have a punching match with a Tyranid, since we see in the finishers, our Marines able to disassemble the xenos rather effectively with just hands. Another mannerism of subclass could be giving Bulwark or Heavy the Boltstorm/Flamestorm Gauntlets, sure we wouldn't be able to carry the shield or a side arm, or for Heavy's concern, a primary and secondary, but said gauntlets are melee with ranged additions. It would allow for a different way to play the classes that way we can have more cinematic goofiness with punching out enemies. Could also give players access to The Stormbolter, as a heavy hitter for some classes like Bulwark.
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Another mannerism of alternative ways to play certain classes, given Tactical, and Vanguard the Astartes Shotgun. It would give some players the satisfaction of hunting Tyranids with a shotgun, and offer some the enjoyment of getting closer instead of further away from Extremis or Terminus entities because some have the mental thought of Invulnerability when offered a sweeper of hordes.