I am sure this has been discussed on here before, and I know other clubs have such a scheme, but surely the answer is an Away Season Ticket, which gives holders priority for buying away tickets. Priority for buying the Away Season Ticket (you could sell, say 5,000 of them) would go to Home ST holders, but any left after a certain time would go for general sale. So when selling tickets for away games, Away ST holders get priority, then general sale. Home ST's could be slightly cheaper, with the money being made up by the sale of Away ST's.
This gives Home ST holders the choice of whether to buy an Away ST, or take pot luck if they only want to go to a few away games. Also, it means that Home ST holders who never go to away games don't have to pay for the priority position they never take advantage of.