Okay, I got that in Halo 4 they made the distinction between the sentinels who serve the Librarian and are Builder caste constructs; which lean towards being more mechanical. Versus the Prometheans and Warrior Caste constructs under the Didact who are more humanoid in their general look. So they decided to keep the two separate. Fine, it makes sense in that game.
But in Halo 5…it really doesn’t make any sense. As far as we know Cortana has taken over most Forerunner technology in the galaxy, not just warrior servant technology, and isn’t it implied that sentinels are the most common type of Forerunner construct? Machines meant to guard over Forerunner sites as opposed to the more unique and esoteric Knights? So, shouldn’t we encounter lots of them? Wouldn’t they be guarding the Guardians
and standing sentinel over them? I assumed that the Prometheans were more like a field army and not tasked with guarding Forerunner sites which is a role for a mass produced construct like the sentinel. That is their job. 
I mean gameplay wise I don’t see why it wouldn’t work. The Watcher is similar, but it is a purely support unit, whereas the sentinel is a gun platform designed to augment knights and soldiers on the ground. Plus if they gave them both long range beam guns and some of the more powerful weapons like Binary rifles they could excel in that role quite well. Also it would be more visually striking to fight an army of floating and teleporting machines.
Plus as I alluded to above, if you are going in the direction of AI rebellion then you might as well add in more clearly robotic units.
Plus fan service. Been in since first game. Should probably stay in.
Not to mention adding in the big sentinels from Halo 2 which were really cool to fight and would be a perfect boss type unit.
I don’t think they even need a redesign or anything since they are floaty and chrome like most Promethean units. Add some orange lights if you really need to but they’re pretty much good to go.
edit: also lots of long range beam weapons would discourage players standing back and battle rifling the soldiers and knights; which is their major weakness. So they would be sort of a tougher flying version of the crawler with access to longer range guns at the cost of speed.
