Mm, wouldn't want them to add something that is essentially buy this dlc to beat the game easily, though maybe the DLC should be scaled to whatever level it is supposed to be done at, so say the DLC is intended to be level 12 there could be +3 stuff there BUT due to how high of a level it is, it would be damn hard for a low level character to get that stuff without ballsy creativity and luck, which would probably mean that they could have done any encounter without a +3 weapon.
Dunno ...
I had in mind Dragon Age: Inquision when i writed that ... there was quite nice crafting system, where all armors you learned were just blue prints and was as strong as the materials you used for them ... so naturaly, as you progressed you get to more rare materials, and potentialy could create better armors and weapons.
And then DLC came up ... sudently all blue prints from that DLC was more powerfull with same material used. :-/
And even funier was when another DLC came up ... and the same story repeated itself. :-/
So if you have max +2 weapons in base game, and you wish to add +3 weapons to DLC ...
I believe you should either re-design rest of the base game, so people need that +3 weapons to finish it, or even better put that DLC after main story is finished.