I would like something that incentivizes people to fight more frequently. I like the idea of attackers/defenders having some sort of respawn location so that there are more fights and less headache regrouping. What I hate about sieges right now is the amount of time you spend running around/waiting, which far surpasses the time you spend actually fighting and having fun.
If people don't like the runestone thing for daily reasons, persoanlly idc, then I think you should be able to mark runes inside holdings but only use them if the city is under siege otherwise the 500m thing still applies. If it were up to me I'd just completely remove it, since the reason it was added I'm pretty sure was because before runebooks could mark anywhere and therefore it was a stop to people having free multiple porting into enemy citys but now that this has changed I think it's pointless having the 500m limit for marking a rune.
In the context of sieging, this can be achieved through the unused political / diplomacy system. If the person who ganked you is from either the attacking or defending clan, you respawn with 90 max in each stat + something else along the lines of a debuff that is clearly debilitating for a duration - Maybe all incoming damage is increased by 15% This stops swarming because, if the time it takes you to return to the battle is longer than the debuff duration, then clearly you are unaffected by it. If you are back in the fight and you have the debuff, then clearly you are respawning very close by constantly. You can still contribute a lot (especially in DF), but the constant wall of defenders that a winning attacking side can face sometimes, will start to go down quicker.
Respawning with 90 in each stat isnt debilitating and neither is 15% dmg taken increased. People forget that there used to be at least a 30second load screen when respawning at a bindstone, then about a year or 2 ago BPG decrease load screen timers. Thus making the death/regearing/back into the fight timer to decrease. I said a debuff that Renders a Character Useless and even preventing from regearing from graves/bank, not a slight debuff. Frog's idea of just an increased load screen timer when being ganked by a player is the most simple for them to code and easiest to implement for a quick fix to the issue.
I dont know if I agree with someone being punished by being locked out of the game for an artificially created length of time. But yeah, a debuff that allows them to return (thinking they can still defend) but heavily impedes their damage output. If they receive extra damage incoming too, they'll be detrimental to a team asking for heals if they get low. They can still defend their holding but get crippled when it gets to the point they are being pushed hard and losing. Most people will just not put gear on in a situation like that, knowing your whole force is overrun. Attackers can still feel better about themselves pushing through respawn swarming.