I like the addition, but keep them limited. And remember why you added a buff like Titan Doubler. As I recall it this was to encourage more players to hunt the titans by shortening the time to spent on the hunt.
And I think that worked quite well.
Perhaps a bit too well, as for certain titans the value of the stamina needed to kill it goes beyond the value of the item that can be obtained. Ofcourse that is not the most important, but for many players it needs to be worthwhile otherwise they will spend there stamina in another way.
These invisible titans need so much stamina spend on them, that the items given would really have to be awesome. (+10 stam gain or something) Cause, why else would one bother?
If the objective is to have more competition on the titans and keep more value on their items, a better way prolly is to release a couple 100 new ones. So the same ones are spent a lot less often. But I imagine that puts a great stress on the graphic department. But it will work better than invisible titans in my opinion. I can already forsee the complaining that the stats of the item do not weigh up to the effort. And then you will have a couple of extra titans nobody wants to hunt.
Don't take this as an attack. as said I like the idea, but I see problems aswell, that prolly can only be countered by adding über-overpowered gear. And that should be the last thing we want not?