Firstly id like to mention i mainly play 3v3 and enjoy using elites that look and sound cool while also matching my armies color palette. That being said i feel that heavy cover makes ranged units way to strong and having the shield and turret be armored limits what units can win the game for your team.
Heavy Cover and Shield Generator Cover
Currently heavy cover provides too big of a buff to ranged units making devastators, lascannon devastators, loota boys and/or ranger, scout snipers units nearly unkillable. By making these units immune to all damage, specifically aoe damage it is possible to cram as many units as possible into the heavy cover without any worry as these locations become nearly unassailable (sure assault marines and certain abilities with a huge range can work but that doesnt feel fun and the latter takes a long amount of time). this issue is very apparent on the 3v3 map Crucible of Vuul where heavy cover is located through out the entire map guarding the only routes to advance into your opponents' base as well as a majority of the nods on the map.
If capturing heavy cover would instead be those units "getting into a defensible position within the heavy cover" and gain 50% damage reduction from all damage while the other 50% would go into damaging the heavy cover this would feel like those units aren't unkillable and would technically add a cover system. This can either be done for each unit individually as they enter and remain inside without attacking for X sec they gain the 50% damage reduction buff, or once captured all allied units that enter receive this buff.
Also i believe setting a unit to melee attack should allow them to enter the heavy cover.
For the shield generator an issue i have is getting it to a low amount of hp only for an enemy unit to capture it giving it a 2nd health bar. i think it should instead allow to be captured for cover similar to heavy cover change i listed above, however with a small change of that the shield gen may still be attacked as normal (for full damage) and the shield's heavy cover would have its own separate fixed amount of hp that doesn't regen.
The Shield Generator Turret and Core
With how the game works if an enemy/enemy team is massing infantry and you counter with anti infantry you should be able to press that advantage either by taking points (which is possible) but also by taking objectives. it is for this reason i think the shield generators, turrets and the core should receive full damage from all attack types. this will make more units and builds viable options (focusing on orc infantry which is something i like to).
Hero Level Up Doctrines
To add more build options i feel the available doctrines a hero can select should be a doctrine for when they are on the field and one empty doctrine slot; both these options should be available when the hero is lvl 1. The current secondary doctrine heroes get would instead be unlockable for 50 skulls just as normal doctrines. meaning leveling a hero only gets you skulls, a player icon, paint color, mastery skin, and fancy circle under character. having more options for doctrines does allow more build options but also for certain elites that look or sounds cool that have 2 undesirable doctrines you can know grab a doctrine that u will actually benefit your play style.