Here's how i think Garden Warfare 3 can develop PvE further
First things first, i know GW3 ain't coming, but we can still have ideas. In general, i think that the pvz shooters have a lot of PvE potential. I know the garden warfare games are more pvp focused, but with pvz there is so much you can do with a horde shooter or horde mode.
Garden Ops and Graveyard Ops already gave us a glimpse into what it could look like. Ops could have been expanded upon in many ways, but nope, Ops was watered down in GW2 (when you pay attention to the smaller details like enemy ai changes, worse quest waves, and overrall easier difficulty, it's worse than GW1), and watered down further in BFN. So we went backwords instead of foward.
While the campaign of GW2 and BFN were solid efforts to expand upon PvE (especially BFN final bosses were cool), i personally feel like that they aren't gameplay dense and aren't super replayable.
So Here's how i would expand upon the PvE in Garden Warfare 3.
You have three PvE modes Co-op Campaign for plants or zombies, rename Garden Ops into Farm Ops, and rename Graveyard Ops into cemetery Ops. i would also make PvE and PvP balancing and design seperated like in Space Marine 2.
Co-Op Campaign: instead of a typical gimmicky campaign like the previous games, it would be more of an action-linear campaign similar to L4D, WWZ, Vermintide, Darktide, and Space Marine 2. Maps will vary in objectives like Deep rock galactic, some maps might be more open like in Helldivers 2 or in the newly released Toxic Commando. The game should use an ai director like L4D to control enemy and item spawns. The Enemy design would be much more complex like other horde shooters. The Weeds and spawnable zombies would be like the common infected; there's a lot more variety of them. it's also worth mentioning that i think GW3 should have a LOT more enemies on screen like WWZ or Darktide. the ai plant and zombie heroes are like the elites and majoris from Darktide and space marine 2. The game will have a new enemy type called specials, which you know what they mean. the bosses from the previous games will still be mini-bosses like the tank. And some missions will have final bosses like in BFN.
Farm Ops: For starters, i would expanded upon the tower defense elements of the mode; taking more inspiration from World War Z's horde mode, Gears 3-5 horde mode, and even dungeon defenders. You can play up to 10 waves or endless. Just like the games i mentioned before, inbetween each wave, you get a certain amount of sun to go to a station to grow your plants. You can use potted plants (that are much stronger than GW1 due to expanded enemy design), defenses like wall-nuts, potato mines, spikeweeds, and even plants like flak cannon or Cob cannon.
Cemetery Ops: Now i don't like the fact that in GW2 and BFN, the zombies are on the defense and the plants are on the offense. To me it should remain the exact opposite just like in GW1 to keep the teams distinct and unqiue. So Cemetery Ops in GW3, would be more holdout and movement based like Killing floor, call of duty zombies, Left 4 dead's survival mode, or Space Marine 2's siege mode. Inbetween each wave, you'll get a certain number of brainz to upgrade your weapons and abilites or buy more units to help you conquer.
I don't want to go too much in-depth, but thats how i would expand upon the PvE.