I would say that introducing any kind of a party vote would be a change from what we do now, which is vote for a local candidate. If you were to move to a mixed member proportional system, under which you vote in the district and then you also vote for your party overall, or if you were to move to a list PR system where the vote is simply cast for a party, then third-party voices, as you've described, would become very important. They could direct their campaigns at an actual ballot or an actual ballot decision that doesn't have any kind of link to that local sense. That kind of set-up would be something that would need to be addressed.
What I would say about list construction is that I'm less concerned about third party financing in how the lists are constructed and more concerned about the informal barriers that already exist inside political parties, because parties draft the lists.