@Mythril sword/knife thing: These are the suggestions that came in mind for the bayonet.
Add a kinamp so that you can use it as anti-armour:
You stab the battlesuit, then press the kinamp activation button to create a shockwave. (Something that strikes the kinamp to create the shockwave should also be added.) That way, you stab the battlesuit, then press the button to blow it apart. It's usefulness however is debatable, since you could probably achieve similar, if weakened results via normal kinamp punch.
Actually do away with the whole "sword" thing and turn it into a really long needle:
That way, you stab something, inject the substance of your choice and you're done. You're facing a human? Stab him and fill him full of high pressure air. That'd be like an explosion going off inside him. You're facing a battlesuit? Give it an injection of clf3. You're facing a door to a bunker? Give it a large injection of clf3. That ought to kill everything in there. Or better yet, find some sort of relatively inexpensive liquid explosive you can inject to things. That way you can use it to open doors without much fear by blowing up a man-shaped hole in them via controlled injections. Of course, this has the downside of being more fragile and very hard to use properly.
Set the sword on fire:
No reason. Just because everything is better when on fire.
Seriously though, if you make the blade very hot (as in insanely hot, it should be able to handle it, it's nigh-invulnerable after all) then the moment you penetrate the thermal shielding provided by a MK1 armour or similar, you can pretty much burn everything with your miniature sun. The downside is that it doesn't really solve the "can't penetrate a battlesuit" problem (although it can probably do wonders on its joints and eyes) and that it takes a long time to activate and deactivate (which means danger of being caught of guard and danger of self-injury respectively) and that it's much more visible than a regular knife. But hey, if you can make them glow a certain colour, you could sell them as lightsabres.
Create some sort of reusable ranged weapon based on the design to justify the cost:
Probably wouldn't work, since there are gauss rounds that can do the same and more for the same price. Still, a mythril crossbow would probably look badass If it doesn't end up being totally ridiculously silly.
@sambojin: Generators are theoretically infinite. They'll keep the capacitors filled forever.
There is, however, an issue. You need a 4TPU generator to run a laser rifle but it's a tad expensive. If you want to save money, you could get a smaller generator (e.g 2 TPU) to save money, however that would mean that the laser would turn into a pulse laser. Instead of firing continuously, it would fire for a while (until its capacitors discharge, time depends on intensity setting) and then it would have to recharge.
So, if money is an issue, you might want to check that in tinker. Otherwise, just buy a 4TPU generator and you'll be set until you die.