Off the top of my head:
-Not playing the same old teams 4+ times a season.
- accommodate for 'big' teams who are locked out of the SPL such as dunfermline, Falkirk, Dundee, Dundee utd, ICT who would not be seen as diddy teams if they were in the SPL.
- allow other teams freedom to develop without the constant threat of impending relegation (both Dundee and Edinburgh have been completely unrepresented in the SPL in recent years which in my option is ridiculous! I'm not saying it shouldn't be allowed to happen, but when our league structure actively encourages it, somethings wrong as relegation set each club back decades.
-more local derbies being played in the SPL, and greater intensity on the ones we already have as they become rarer.
- make the league closer with rivals to the old firm only having to play them 4 not 8 times providing increase excitement for fans and interest from television companies.
- give more youth players a chance to impress on the big stage, attracting interest from scouts and national team selectors when seen to be performing at a top level.
18 top leagues blooding youths. 18 teams where you can be playing at the highest level. More kids are gonna look at football as a serious career choice and continue to pursue it, rather than getting jaded and jacking it it by seeing no path to the first team at the big teams or playing at a diddy level where scouts havn't visited in decades.
- allow space for ambitious young teams to build and expand into the top league (e.g. Cove, Edinburgh City) without consigning current SPL teams to relegation, obscurity and death.
I don't think any outsiders in a few years time would have any more worries about league quality looking at this table than they do already looking at the current one:
Rangers
Aberdeen
Celtic
Cove Rangers
Dundee
Dundee utd
Dunfirmline
Edinburgh City
Falkirk
Hamilton
Hearts
Hibernian
Inverness CT
Kimarnock
Livingston
Motherwell
Partick Thistle
Ross County
St johnstone
St mirren
Thats 20 teams there who even Considerring annual promotions and relegation would most likely comprise a 18 team top flight annually. Genuinely, given a few years to adjust to the top flight don't think I'd blink at any team playing in the top flight (though some are obviously going to be contenders for the bottom of the table).
Add in the occasional cameo from Ayr Utd, Queen of the South, Raith, airdreeonians (who's fans would liven things up by just being extatically pleased to be in the top league) or of course the emergence of more ambitious teams through the pyramid such as Bonnyrigg rose, Kelty hearts, spartans or Brora. This would see top quality football being played further afield, being enjoyed by more of the population, revitalise the fan base of teams locked out of the SPL, and refresh the league in general.
It might not work... But if it Did it would have the potential to really improve our game and product. Status quo... I fear we will just continue to rot that our game has been experiencing since the 70s.