I think sprint would work in campaign if the battle areas were wider with better lay-outs and Elites could run after you. That and get rid of bullet sponge Prometheans. Also bring back soldiers that have perma death. The squad AI in Halo 5 was attrocious and I hated that they kept respawning.
I really don't think smaller spaces are hurt by sprint either. Especially how they used it in 5 where your shield won't recharge while sprinting. Having the variable speeds makes things more dynamic, and I love the cat and mouse game-play that didn't exist before. The older games had these large levels that really got a bit "sloggy" due to the plodding nature. Obviously, new efforts need to be made to compensate for a faster speed, and vehicles need to be reformatted to be useful, but I like the IDEA that the Spartans are so capable that a vehicle isn't as necessary.
I agree that the enemy AI needs a bump in skill in order to be more of a threat. I miss the additions from REACH like the jumping jackals.
Bottom line is that I think 343 did a marvelous job on the base Spartan gameplay. Now they really need to focus on bringing up the rest of the game. It is worth noting that 343 very deliberately wanted to get Halo to 60fps. They don't get enough credit for that.