First, Thank you for communicating with us! Involving the community and our feedback is greatly appreciated!
Now, unto my oppinions:
This one seems popular considering the previuos replies to this thread.
I understand the “not defending when you need it the most” argument, but I feel it’s a decent compromise. (a little more on this later)
IMO this is unnecesary if the next one is applied:
If the goal of this is for Towers to be less threatening, you could also acomplish that by lowering or removing the armor reduction component of tower shots, and not necesarily the one of fort/keep shots.
This would also avoid the problem with having 3 things shreding your armor at the same time, as getting to cap armor reduction means you’ve been fighting for a long time under a fort/keep (and arguably should be dead by then 
)
Before “call for help”, the fort/keep had slow and towers didn’t, so this doesn’t seem to me like it adds complexity or is unintuitive (different structures having attacks with different effects seems perfectly reasonable for a game like this IMO).
So my tought is that towers should lose armor shred, and the targeting system should be changed to work as follows:
Towers and Fort/Keeps have a priority for targeting enemies:
Minions > Mercs > Heroic Summons > Heroes > Map Objectives.
But the priority is reversed if something attacks an allied hero in their range.
So if a merc hits a hero, the structure would target it even if there are minions around, and so on.
Thus, you do make structures prioritize objectives, but only if the objective atacked a hero, allowing the attackers to decide if they want the obj or the heroes to tank structures according to their strategy, but also allowing the defender to try and force one or the other to be targeted.
The reason for separating heroic summons is two-fold:
for one thing, others have stated how they find it frustrating to get attacked because of an entity they can’t control, such as Anub’s Beetles. With this change, the fort would attack the Beetle, thus still allowing Anub to strike at structures, but not to dive heroes under forts with impunity, because then he’d be prioritized over his summons.
The other reason is that I think it’s more intuitive: If Zagara is on the other side of my gate and drops a pod, and the roach starts attacking me, I want the structures to defend me from the roach, not to focus on Zagara, 'cause it’s the roach that’s triying to kill me! (even more so for Hunter Killer, 'cause that one could kill me ).