My suggestion is simply that members of a family are proed for a very short amount of time when any upper of that family is killed, members of the district are proed for a short time when the GF is killed, and the whole city is proed when the GFC is killed. 

By very short I mean 5-15 minutes. This could scale depending on the rank of who is killed i.e. LHM gets shot and the family is proed for 5 minutes, when the CL is shot it's 15 minutes. 

First strikes and aggression would still be extremely effective, with the ability to take down every CL in the city if you shot GF's in the right order. It may involve slightly more coordination that it does currently but I don't think that is too much to ask for the reward of taking down a city. The main benefit is that it would offer crews which are prepared and active the chance to organise pro shots and provide some resistance, or at least make their last moments exciting.

In game justification for this would simply be that if you hear about your CL/GF/RH getting shot then you would probably go to ground for a while to figure out what's happening. 

I'm not proposing this out of any sense of fairness since this is the mafia and things aren't supposed to be fair. I just think that war is an exciting and integral part of the game, and currently they all end in about 30 seconds. For all the hours people spend grinding for guns/BGs I think it's in the game's interest to encourage take downs and wars to last a little longer and be more exciting.

There would have to be some minimum amount of time that the dying upper had held their post prior to getting shot for the effect to trigger - maybe 5 days?

In the Mafia, all is fair in love and war. In the Mafia, when there is friction, the families, (not just the Don and/or CL, UB or GF)  involved are all made aware of said friction before guns go a blazing. If a war ends in 30 seconds, it is not a war. 

This is not a solution to wipeouts.

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Part of the reason I suggested this was because I felt like it would have a much bigger effect on larger groups taking down smaller ones than it would in the reverse scenario. At least that's how it plays out when I think about it, but perhaps I'm wrong.

Scenarios:

1) Current system: Every made man+ in the district is killed probably within 1 minute.

With auto protection: Attackers could either shoot the most threatening member of the upper structure first (likely CL), wait for the the protection to end on the other members then begin killing them. This would give the small district a chance to organise pro shots/travel/shoot back.

Or the attacker could take out as many threats outside the upper structure as possible and then hit the CL/RH/LH. This options gives a small chance for the uppers to react (travel) if they are very quick when their members start getting shot, and would leave two uppers alive with a short pro even in the best case scenario for the attackers. 

In the event that one of the attackers mistakenly shot someone out of order for example a LHM or maybe even the GF if the district has one it could give an even greater opportunity for the small district to react.

2) Imagining the underdog district has 4-5 cap killers and the city they are shooting at has minimum 6 crews.

Current system: Most likely the underdog district will use all of their guns to shoot CLs/GFs in the first wave, since it causes the most confusion and the CLs likely have the biggest guns. Then the lower guns open fire on survivors.

With auto protection: The underdog district can still shoot all the CLs/GFs they want in the first wave as long as they shoot CLs > GFs > GFC. Opening fire on survivors by the lower guns could either wait for the auto pro to run out or be targeted at any of the crews which weren't hit in the first wave.

3) If they are a single rogue auto protection would basically have no effect at all on them unless they perk since the pro would run out before they could shoot again anyway. If there were two or more rogues they could simply shoot people from different cities/ districts if they even have the guns to hit GFs or GFCs. If they really need to hit the same city it could just be GF > GFC and they would be unaffected by the pro. 

I think the effect on underdogs would be minimal since they will generally be spreading their targets between crews and districts, meanwhile a more powerful attacker looking to wipe out a single crew/district would not be able to bring all their guns to bear as quickly as they currently can.