For me the issue with the SPL as it stands is boredom. Playing each other 4-6 times a season is ludicrous, by the time we're playing Kilmarnock for the 4th time in a season I'm starting to lose the will to live.
I don't get the idea of meaningless games personally, all Hibs games have meaning so I don't get that arguement against large leagues. I don't follow Hibs to watch them win the league (or I'd have given up a long time ago), I follow them to watch them play football.
I'd go for a 16 team league playing each other twice. 16 teams would be the optimum number for me as I don't think the Scottish game could support 18 top flight teams and still have a meaningful first division. 16 team first division as well and then split into regions below that.
Playing each team only once home and away would add interest IMO. Most fans only go to home games so would therefore only have one chance to see each team. This could add impetus for people to actually go to games. Generally most teams will have something vaguely interesting going for them that might tempt people along to see them. it would also make rivalries much more intense.
Make up the shortfall in games with a 32 team league cup split into seeded 4 team sections, as described by others above. This gives you another 6 guaranteed games leaving you a minimum of 36 games a season, 18 home 18 away.
Split all tv monies equally (or ditch the Sky deal altogether).
Then move it to the Summer.