In my hypothetical scenario we'd be going in with only three overseas players anyways so Pretorius could come in. I do agree that teams will seek to just extend their batting orders or have an extra bowling option in place of a batter but I feel that is a fairly simplistic way to use this rule.. it's much better utilized in maximizing match-ups and creating unfavourable situations or having the benefit of a bowler who can do the job in a different way after the previous one is done. There's still a lot of other ways to use this rule that I'm probably not even imagining.. @Aislabie may have some good ideas.
Yeah this is absolutely my jam - I've been thinking about ways to implement substitutions into T20 cricket for years.
It would be much easier if they allowed teams to swap an overseas player for another overseas player, but the challenge would doubtless be part of the fun. And there's more than enough Indian talent to go around.
I would absolutely not look to do anything for the batting (although if it came to it, I would include a batter in my subs pool just in case there was a spectacular first innings collapse. However, my main plan would be to use it to cover bowling options. I would have a spare pace bowler and a spare spin bowler to cover for injuries that might happen in the first ten overs or so of the fielding effort. But my main goal would be to include a bowler to specialise in bowling death overs - all four of their quota in the last 10. If none of your starting XI is a suitable back-up keeper it might be worth including one on the bench, but I'm not completely sure whether the IPL wishes for all substitute fielders to come from the subs bench or not. It would be sensible if they did, but this might be a loophole.
So something like:
Mayank Agarwal- Rahul Tripathi
- Glenn Phillips
- Harry Brook
- Abhishek Sharma
- Abdul Samad
- Washington Sundar
- Rashid Khan
- Marco Jansen
- Bhuvneshwar Kumar
- Umran Malik
- Samarth Vyas
- Mayank Markande
- T. Natarajan
- Kartik Tyagi
I've picked this on the basis that your back-up keeper does not have to come off the bench.
However, I do have a bolder selection if anyone feels particularly brave:
- Start Rashid Khan on the bench.
That definitely seems insane, but think about it:
if you bat first, then you just sub him in for whichever of your opening batters gets out first, and you immediately get a bonus death hitter as well as all of his overs with the ball.
If you bowl first, then your designated new-ball bowler (presumably Jansen) can bowl out in the first half of the innings, and then you replace him with
a full quota of Rashid Khan.
Of course, if you need to have all sub fielders coming off the bench, then you'll need to have a decent back-up gloveman in your starting XI to cover for that particular emergency.
Also, Sunrisers are absolutely
STACKED, what a squad.
My only worry is that we have no backup to Dhoni. There's no specialist keeper in the team and he'll be playing the IPL yet again after a year of hiatus. I don't want to rely on Rayudu or be forced into playing Conway and upsetting the team balance if we happen to miss him for a few games. I wonder how we'll exploit the impact player rule too... our squad filled with all-rounders is perhaps the one that least needs it. Maybe we could just use it to have Mukesh and Chahar swing the ball in the powerplay and then swap Mukesh out for Pretorius if we're chasing. 
