Agreed on limited immunities being a potentially useful mechanic, though finding ways to limit corp-hopping for wardec initiators would be necessary in a targeted system, what with shadow wardecs and shadow corps being the issue they are. If you want to see some hilarity, look at the corp histories for pilots in the Hogs Collective to see how that all shakes out. The question is how we balance cost/difficulty of the wardec to make it more than peanuts, but also to prevent it from being easily (or cheaply) gamed. I wonder if a 'forfeit' or 'deposit' system might be worthwhile? For instance, the attacker has to essentially 'bid' for their victory conditions; if they win, the deposit's largely returned and they've got their killmails, etc; if they lose, a part of the deposit is given to the defender. Wardec costs would then scale with bids, but also put direct incentive to pursue completion of objectives for both sides.
Alternatively, I wonder if limited wardecs might be a thing: region limited, or ship-class-limited, sub-week durations, and so on.
I'll admit, though, that my mixed feelings on the wardec eligibility component still tend toward it being a not-terrible idea. As it stands, there's essentially no barrier at present for a wardec initiator to make highsec extremely dangerous, indefinitely, at little cost. The trick, for me, is that eligibility needs to be more broadly founded. Imagine casus belli functioning sort of like killrights, excepting you get them if, say, the person tries to flip your can, or tries to gank you, or kills you in lowsec, etc. So, PIE'd have plenty of reasons for people to wardec them after lowsec roams, provided the parties then sell those casus belli permissions. Hell, I'd love a pseudo-FW where corps can 'declare allegiance,' thereby opening themselves to wardecs even if they don't go full FW.
Of course, the PIE example does sidestep one thing: it's easy to end up in a war with PIE, even with the bandaid system. Join GalMil or MinMil. This, I think, highlights one element of the philosophies clashing here. In the case of PIE, for instance, "But they're my enemy!" is indeed exactly reasonable, and the game already provides mechanics for it. For that matter, no mechanics are needed to pew with PIE or other lowsec PvP groups in their notional areas of operation.
Honestly, in my view the issue one of the overarching issues is that we need better kiddie pools for introducing new players to pew; highsec wardecs are so mechanically flawed that, between that and ganking, highsec paradoxically ends up being surprisingly dangerous by comparison with some sections of nullsec.
Also, agreed with Silas, essentially.