Draco
Adida
The key to my statement was "start with," as in the UNSC would begin by compartmentalizing the design that was already successful even further (see how the Spartan Laser was something that started as a vehicle based weapon for the Sparrowhawk in canon that was later miniaturized for troop use). So you could have something like a DMR or sniper rifle weapon that had a short charge time to fire a finer beam more suited to infantry specific combat than anti-vehicle, or perhaps a laser turret of some kind. Basically you would want to show a progression of UNSC development of energy weapons in a natural manner, and the best way to do that is to show things derived from what we already know, and then from there introduce more novel weapons as the games continued.
I never said the Flood shouldn't have been left a mystery, I myself find the ideas of a lot of the stuff in the Forerunner saga to be lackluster/too direct in their explanation of what occurred (especially ancient Humanity, I don't know what Greg Bear was thinking doing that plotline). But saying we've beaten the Flood in five games as a reason they shouldn't reappear when you'd also have these hypothetical future games where you'd fight some variation of the Covenant again anyway makes the sentiment ring somewhat hollow.
While Halo 5's plot certainly came off as "evil robots," that doesn't necessarily mean that the logic plague would come off as that on its own. Mendicant Bias, for example, was infected by the logic plague, but that story didn't seem like it was written as a evil robot story (at least not in the Halo 3 terminals). Rather it enhanced the threat of the Flood because it was able to turn someone to its side without physically infecting them via biomass. That would enhance the threat it poses, which to some extent was done with the introduction of the Gravemind in Halo 2; no longer was it a faceless mob of John Carpenter monsters, but an intelligent force that could properly plan its goals to devour everything.
Ultimately something like the logic plague makes the Flood threat more multifaceted, enhancing the depth of the fear that was already present with such a malicious and clever force. It is not restricted to one vector of attack, it is something deeper that can corrupt a polity or species in many ways, making it even harder to stop.
While a fair point, the various Flood containment facilities on the other Halo rings are something that will need to be dealt with eventually. They're not going to go away on their own.
That's a fairly difficult concept to realize when the Forerunners had already been established as a galaxy spanning empire, and a single Keyship (which wasn't even a Forerunner warship) could tank Super MAC rounds like they were spitballs. They'd have had hundreds of research stations and military bases across space in order to properly police and observe the different species during their reign. Any one such base would be a monumental discovery that would cause a massive power shift for whoever controlled it. It doesn't have to be a Halo for it to be a big awesome artifact.
