Engineering wise:
I think with absorbency I get and agree with the comments saying they don't need to be as absorbent as the crazy tape up diapers now available, but a good capacity is still needed, if you are aiming at the ABDL market rather than IC users then higher capacity than what is currently available in medical brand pull ups definitely. No matter how much an ABDL person might understand that pull ups are meant for smaller amounts they are still likely to end up flooding them.
I think high standing Leak guards on the sides would be important for this as pull ups do tend to sag more because of the elastics and that should help contain things. I also think cloth backed is much more appropriate than plastic backed, pull ups are meant to feel/seem more like big kid pants so smooth plastic outer should not be used. However something that will still crinkle and rustle a bit would be good.
Potentially slightly thicker sides and although I imagine it is patented, hook and loop easy open sides something similar to these.
If done right this could actually really help with adult pull ups much more than children's because it means that when they are a bit wet and getting heavier, if they start to sag you could potentially tighten up the sides by reconnecting it further back.
Some brands of children's pull up have a little tape on the back that you can use to secure a wet rolled up one when disposing, this would be nice to see.
Potentially male and female variations with more of the absorbent materials in the right areas for either gender, might help them work a bit better but the print designs would have to be available in both to not upset people so that could be a pain.
Wetness indication, anyway would be fine but a fading print is best as it makes you want to change to get the picture back.
Aesthetic wise:
A range of prints suitable for any "age", but keep it simple, too many companies go over the top trying to fill the outer with all sorts. Though one important design feature I feel has to be the work "back" on the back side, most children's pull ups I see have this to help them getting used to dressing themselves.
Things like this for the "youngest" littles...
To this for slightly older littles (I know nemo and pooh are copyright but other characters would do)
To These for the middles and older littles.