I find it surprising that you are willling to invest your time Dave, but if you dedicated yourself to that task, well, I can only wish you good luck.
Warband still has a solid online playerbase, wiping the shit out of it coupled with some new/old xp gain system might bring some lost souls.
There were some decent suggestions in the past about new xp that promotes a bit more teamwork, multiplier is a failure.
I was thinking about something like that:
The proximity xp/gold as a bonus to gain based on victory/lose condition would work better I guess.
Let's say that avarage round last around 3 minutes, that gives 4 tickets in current system, with avarage gain per tick I guess around 8k xp, it gives 30k xp per round. So if you win a round on a battle/siege you get 30k if you lose half of that, on top of that there is a bonus from proximity xp system, like up to 20% of round bonus. Numbers are just example.
That is how I see it, no tickets based on time to speed up gameplay, mixing it with proximity system promotes sticking with team.
Both current multiplier and xp barn are a different case of autistic behaviours on servers.
Numbers are just an example.
I'd also consider cutting bonus skill/attribute points from leveling up after lvl 30, instead give a heirloom/upgrade point for hitting next lvl. But that requires too much of a work I guess. The reason for it is that in my humble opinion havng advantage from both higher levels and heirlooms is just too much. Of course it's still up to personal combat experience/skill but you can't deny that it makes difference.
That cuts it, also it encourage players to keep playing as the keep the getting the rewards from leveling up. Might be also a huge relief to balancing the items, but heck who is going to do that