I think they should take the good things from both to make the perfect Halo game. I don't think the DMR should be taken out, because it's no longer overpowered in Halo 4. Just keep all the weapons the way they are in Halo 4 and It'll work well. Because some people really like the DMR and getting rid of it might make people lose interest. I think they should take gametypes from Halo 4 (Dominion, Extraction ad Regicide and maybe Infinity just because some people like it) include Classic gametypes and Trueskill Ranking system (For Halo 3 fans) and take the Custom Games features and Firefight from Halo Reach, as well as maybe Invasion. Invasion maps were a LOT of fun to build in Forge.
Also, the Forge map should be like this; Lands with terrain and lots of detail, e.g. Waterfalls and caves (Like Forge World) and separate flat lands (Like Forge Island) and maybe they could have other terrains as well, like maybe deserts, and snowy lands. New Forge features could include new pieces, placeable terrain (Could be used on the flat islands) and decorative terrain (Like waterfalls) and maybe a Time of Day setting.
My vision of a perfect Halo game