Tekkit will probably not come to Xbox, as it is a pretty big mod-pack, and for stuff like this the Mojang and many others will have to be asked about allowing mods on the Xbox and ALL the Original and Current Modders making the mods including the Tekkit Dev. Team will have to be asked for Permission for use of the Mods. Also, why would they make this for free when it would take 4J Studios a very long time to add all the Custom GUIs, 100s of new blocks and 100s of new items?
On a last note - Wrong forum, this is in the Indie Games forum, and should be situated here: forums.xbox.com/.../2414.aspx