What programming language are you making this in? If it is .net then there will be several opportunities to make it work on platforms other than windows.
If it is in Java; then it will (obviously) run on the majority of operating systems. If it is in something markedly less cross platform: eg visual basic or the wonderful GDI, then there is less hope of porting it to any other system.
Nonetheless, wait until the first release for (whatever operating system har_j1 uses) before demanding support for a different operating system.
Good luck with this, I hope you acheive what you want, and I can't wait to try this system out(whatever form it takes).
Oh no... Not makemyitem or whatever it was called again... the forums were flooded forever with "LOLAWESOMESAUCEBLOCK"s that were the same thing as vanilla stuff with new textures/slightly tweaked durabilities... I don't know java, but I still hate this idea. The forums are flooded enough with those retextures/slightly tweaked power/durability things anyway... just search something like more ores or something to see. We don't need MORE of them.