One thing that i feel is missing from this game. The Corpus have advanced technology and Robots of many types. The infested only take over the humanoid bodies, however.
IMAGINE walking around shooting Infested and being a badass as usual, then you see a group of Corpus Moa coming toward you, much slower, and you notice that they have been taken over with the Plague (EDIT: *ASSUMING* that the plague is a nanite or nanite-based virus, tech should be affected). just picture a normal Moa, but with Infested tissue (assuming a leaper jumped on the moa then fused with it for example) all over it. that would be really cool and it would create a bigger purpose for the infested, infecting more than just 2 different things.
AND another approach, would be to imagine the Grineer, (since they are in battle with the Corpus anyway at times) have taken over one of the Corpus's shipping routes and have been taking shipments of Moa's for themselves. since the Moas are connected with the Corpus, the Grineer have to modify them, by strapping control circuits to their guns for example, removing their shields (to save power for powers like teleporting), and adding Alloy armor plating to them.
you could also go out and just say that the Grineer abandoned their Roller project (I THINK EVERYONE HATES ROLLERS and this is one way they could be removed logically lol) so they instead looked into robotics, and have recommissioned thousands of pre-war robotics from before the Orikon Era. they would be bulky, rusty, crude robots with guns like the Grineer, and even more humorously make it so the Grineer robots have personalities even as simple as yelling in high pitched mechanical voices when being injured.
TL;DR MAKE ROBOTS FOR OTHER FACTIONS