Well I was only going to add 4 or 5 new weapons.
But I dont think your theory makes a difference. Its up to the map maker to place just the right balance of weapons. What you suggested would only apply if:
OH HAI GUYZ, IM GOING TO ADD NORMAL TTT WEAPONS BUUUT ADD IN A NEW GUN THAT ANYONE CAN FIND!
*Compiles map and adds to server*
Player 1 becomes traitor and picks up the only custom weapon.
Player 1 boom head-shots detective and 5 minutes later he joins other players in server in 1 area.
The Whole group notice's he's on the only one with the custom wep.
PROFIT.
Its a matter of balance really. What your saying would have ruined our TTT if we choosen ttt_altborders.
3/4 of weapons in that map are deagles and you'd find maybe 2 shotguns, 1 rifle and m16 and a few para's, and if the player uses another weapon besides the deagle to kill...then it would be a 2/4 chance of him being a traitor.