War, War Never Changes.....
Remember what war is. War is a violent attempt at imposing your will on your opponent, and it is shaped by the constraints available resources and not so much by the wishes of its participants. Wars are won off the management of the hard and the painful, and that can never change.
Many players complain about the sov system and how miserable it is to participate and demand changes. However, no possible modification to the system will make it easy and fun, for the infliction of difficulties and irritation to the opponent is the very strategy that makes victory possible.
Wars are won off resources, and the relevant ones are: money, manpower and skill (intelligence). Whatever the mechanics, war will seek to stretch those resources beyond the limits of the enjoyable or reasonable. It is an unbounded escalation where the group with the greatest inhuman devotion wins.
Activities that are on the whole, "fun" can never be decisive. In a world of immortals it just means the opponent comes back for more and more fights, and strategic victory is never achieved. True victory can only be achieved by inflicting so much misery on the opponent so that they lack the will to gather resources and fight. Defeat starts and ends at surrender of the mind.
1. Lets take a common proposal: make small gangs decide sov~ coz small gangs are fun. Lets say through some convoluted mechanism so that multi-system spanning small fleets are now the winning strategy.
So what happens? Well people would start by running casual gangs and find it not really winning since opponent can match it easily. So they escalates, with fleet composition of ever increasing "faggotry" with whatever falcon/logi/whatever for both maximum tactical advantage and to lower the desire for the opponent in engaging them. If your pure falcon fleet or other trick can piss off enough casuals on the other side to make them not log into the next fleet, you've just won a great victory no matter if you hold grid or not at the end of the day.
Perhaps that is insufficient to win. Then the next step is to escalate on intensity. Around the clock 7/24 ops are run to wear down the opponent, more fleets are formed to contest more systems, and manpower is stretched to the limit as the leaders use every trick in the book to get people to log and overwhelm the other side as hygiene, school, careers, sanity and food intake is sacrificed, and the operation lasts as long as they could be pushed, up to the point of burn out and very pissed off players quitting the misery collectively and have the war end. No matter how much fun a battle could be, when scaled up to the point of player breaking it can no longer be fun.
If the mechanics does not allow the escalation to the point of burn out of one faction, then a perfect defense can be easily mounted and taking sov becomes almost impossible.
2. Since having isk on field decide the fight in the form of capital/superonline is well know, lets examine the last item on the list, that of skill.
Being a sandbox however, "skill" lies not in mere individuals at the front, but the organizational and strategic level operations. People will escalate and it is the features that organization have the most difficulty in emulating that will decide the outcome. What this means is things like spying, metagaming, and boring staff work like keeping track of every pilot, every ship, the organization of logistic networks, command and control structures and such will become more and more important, with ever increasing number of spreadsheets. To win, organizations will have to push closer and closer to the structure of a real military with their their stratified structure with ever increasing emphasis on discipline and professionalism and further and further removed from free form fun that gamers want.
There are other limited resources, like trust, that can be used to determine outcomes, but I don't think they'd be any more enjoyable then alternatives.