I thought it was gnome rogue because you have everything besides alchemy, but someone suggested it should be gnome enchanter so you are able to enchant metals... But I'm not sure how relevant is that on a current live server? How dependant is jewel crafting on enchanting nowadays? Also, how useful you’d judge having tinkering+poisons versus alchemy as a shaman? Thanks!
The way I read you title, "potential", it's gnome rogue, no question. The question your friend answered is "usefulness", and for that, it's a subjective answer, but I can see gnome enchanter being a good answer.
Yes, a gnome rouge is open to everything except alchemy and enchanting components. Sounds like a gnome shaman is needed to fill the gap
Gnome Enchanter all the way. Also work on Jewelcraft and Spell Research. You'll thank me later. A rogue and a shaman can be your alts, you level them up and master their crafts later at your convenience.
You can cover every trade with a Gnome Rogue & any Shaman - but you would still need at least an Enchanter box for enchanting materials to be fully independent, pretty much every trade will need something you can only get from an Enchanter at some point.