I've just finished an Austria game - most fun I've had yet in EU4 - there's lots to do besides blob, and plenty of scope to make gains without having to rush off to colonise the Ivory Coast.
Key things I'd say to play as Austria -
Play on Iron Man. Makes the politics much more tense & enjoyable.
For me - don't centralise into the HRE nation - the Imperial mission tree is rubbish, and the big-grey-blob isn't a fun game - you can play as literally any other nation if you just want a giant nation with a big army. Removing the politics of the HRE removes most of the fun.
Via the below, you'll end up with the Imperial vassal swarm (if you revoke) and can have a great time beating on France / the Ottomans / Spain / Hulk-Denmark.
I had the lot by 1587, which left plenty of time for fun elsewhere in the game.
Diplomatic -
Get Hungary / Bohemia as PUs - use the mission tree / marriages to achieve this if you don't get them at the start.
Using marriages & so on, PU Milan
Don't let the Italians leave via the Shadow kingdom
Take diplomatic ideas - this will make managing the empire much easier, as you'll eventually have 6 diplomats to send on 'Improve Relations' missions
Keep your relations & marriage up with Burgundy & make sure to take all of it if the Succession event goes your way
Keep the Pope in the Empire
Later on, be sure to integrate Hungary ahead of using the Revoke reform, if you go that route
Military -
When it emerges, PU Bavaria
Take Quality ideas - better generals, more tradition
I took Aristocratic ideas also - extra combat bonuses, and another diplomat
Smash up Poland (use the mission tree) to prevent the Commonwealth dominating your eastern border
Use your huge army to grind down the Ottomans & release the Balkan nations into the Empire
When the Reformation kicks off, use your huge (Austria + Hungary + Bohemia + Bavaria) army to nuke all of the centres of reformation
Force Religion on non-Catholic princes during the reformation - especially electors
Expand the Empire CB on bordering countries (Italy is key really)
Build semi-historic fortresses in the Low Countries to keep the French out (the Barrier) - also good for your tradition
Imperial Authority -
Keeping down Reformed & Protestant areas maintains a nice .10 - .20 tick all the way through the Reformation
Lots of PUs / alliances around the Empire reduces wars, which keeps up the IA tick (peace increases it)
Make sure to release princes from your Burgundian lands / Italian lands
Use the 'Demand Unlawful Territory' to keep up the numbers of OPM princes. Try not to cause heretic princes to be released this way.