it would be cool to have dual swords like 2 cutters or something fast like that, or as other people are saying magic would be pretty cool as well, spear? would be longer ranged and maybe could be thrown

To be honest, I'd rather prefer something like an accessory over a new class of weapons. No not like trinkets.
I think the best way to describe it, would be to refer to it as something like a Monster Hunter armor skill, if any of have played that.
For example, here are a few ideas.
Swords
- Explosion occurs at the end of a combo
- Sword attacks have an increased range
Guns
- Extra shot before reloading
- Increased knockback
Bombs
- Larger AoE
- Blast lingers for longer time
General
- Shield can cover teammates when shield button is tapped twice
- Status effects more likely to happen
Of course, a few of the more potent trinkets would bring downsides too.
Some of the weapon related ones would reduce general damage, making using
the effect of the trinket to outdo normal damage important. Perhaps the status
effect booster could increase the effects of statuses on the knight that uses it. I dunno.
I'm really kinda thinking about Team Fortress here. I mean these things
(I just remembered what constitutes an accessory in Spiral Knights) would only be
more effective in certain situations, but those situations could come about often enough that
the trade-off would be worth it.

As a note, I guess it would be nice if there were certain more powerful weapons, like, huger swords,
gatling guns, whatever, that are obviously more powerful, but could only be used so much per stage.
Perhaps unlocking them would involve a mission? I dunno. Oh and I could imagine them taking up
a special weapon slot of their own.

P.S. I typed this all up in a storm, and haven't really thought through these ideas.