I'm new to D&D. I currently have one character built: a human fighter. I've given him the Two-Weapon Fighting (TWF) fighting style, as it sits nicely with his character. I'm currently at Level 3, and intend to take the Dual Wielder feat at Level 4. 

I note that past Level 5, TWF starts to fall behind the Great Weapon Fighting (GWF) fighting style for damage output per round. This doesn't bother me too much but I'd like to keep up with the rest of my group. I have just been granted a wish by drawing the wish card from the Deck of Many Things, and I would like to use it to bridge the gap between the two styles regarding DPR.

I'm thinking of wishing for a pair of magic longswords with a damage modifier that increases per level. I ran over the idea with the DM and he agrees it's a reasonable wish, as I'd like to continue with the character. I am really interested in knowing how I could make this modifier match the DPR from GWF, and I don't want to create anything that will be overpowered in the early stages or underpowered further down the line.