Yea, Halo could use newer enemies for sure, I agree with that. They've changed them a bit it looks like, so the enemies have a bit more variation than before. The ADS (Aim Down Sights) is left optional, so they don't penalize your accuracy for not using it, traditional Halo players will likely not use it except for when zooming is seriously required (DMR for instance). But there are headshots and they are a key element to Halo, as that is what generally separates good from bad players.
I agree about the Alien weapons not being as good, or feeling as good, but they look like they have better combat feel this time around.
Halo and Destiny felt largely the same to be 'gunplay' wise. The only major difference is that Destiny's guns all have more and more personality to them (and variations) , the enemies - not so much. Most are rebadged/reskinned versions of the Vex, Hive, Fallen, and Cabal. Having said that it lines up pretty well with Covenant, Flood, Brutes, and Forerunners. Almost lines up too well. The only difference is that Halo has been running these races for some time now, where you've only played 1 Destiny game. You might get bored of it if you see the same enemies in Destiny 2 --- but I doubt it.. lol it's a solid loot drop, great combat mechanics game.
I imagine for H5 there is 1 unannounced race, I'm going to take a stab and say the Prometheans. You could also be fighting UNSC/ONI as well this time around.
Had to double check Destiny since I haven't played in a while, but something I didn't notice until now is also the lack of blood. Instead blood is replaced by numbers! lol H5 is lacking blood or something that would make the combat feel better when the enemy is hit.