The DMR wasnt the predecessor to the BR, they are two completely different riffles used by the different military branches in the unsc. The army favors the DMR (in reach you are army) where as in the rest of the halo games you are navy and fight alongside marines ( both the navy and marines favor the BR)
The UNSC army force's are the only branch that still use the DMR, you just dont encounter any army troopers in the other games.
I would like to see other weapons from the fiction brought into the games. Attachments for weapons maybe not such a great idea for halo, but a 2ndary fire function like in Turok kinda seems like it would work. Im all for changing the aesthetics of weapons, but this wouldnt work due to halo weapons being scattered around the map, and not picked with class like in call of duty.