You've got to pick up the materials from nearby asteroids and space debris!

Can I have the tileset I will give you the credit for graphics

No need for the color ramp or tileset...by me anyways.

As for gameplay, it seems that it should have something to do with welding in space.
I would say that you probably run around other people's ships fixing up their broken computer equipment, air ducts, flooring, fuselages, and basically anything else that could benefit from welding. All the while torching aliens who happen to get in your way. Collecting random items that upgrade your equipment, refill your energy,  and boost statistics would be a must. Making a way for them to keep upgrades permanent might be cool. Perhaps add a shop feature between levels so one could purchase new stuff for the next. If you want to keep it more action based make the upgrades less permanent (ala mario or something) but more extreme and add a time limit. Also, make it so you can control those robotic arms for one reason or another. Perhaps the only way to destroy aliens is to grab them with the robotic arms and send them into space. Resulting in exploding heads.

Either that or make it strategy based somehow. But I don't like the sound of that for such an intense game as STARWELDER!
Those are my thoughts on the matter.

Thanks for the compliment! I actually offset the walls on purpose (they can tile when aligned, too) to add a little variety to it. If it jumps out at you, it's a valid crit. If there's interest, I can upload the color ramp and tileset.

Any ideas for gameplay?

Amazing palette as usual. I happen to have this 256 color I made up ages ago that seems to have close to the exact colors you have used. Actually I've randomly put in high color google images to see how they're affected in the palette and it does quite well with them even. The point I'm trying to make is I admire your color choices. Even with a fabulous palette I've yet to acquire the ability to piece the colors together so beautifully as you can.

The only crit I can think of in particular is how the rails along the wall jump off by one or two pixels here and there. For being so thin the change is rather drastic and keeping them in a straight line would appear to be more forgiving.