RTD's -
Starting from most general --
1. At the top of the list is affordability - the cost associated is significantly greater adn difficult to justify beyond the sole portability of it. 6packs, 12packs and larger cases should be avaialble with savings associated with the high quantity (obviously).
2. Skip the glass, the soft paper/plastic hybrid container, go with either plastic bottle (Oh yeah for example, or even bottle like endorush, redline, etc). I like the wider mouth, also allows for me to scoop in my own powder, ie ALCAR, creatine, or any other powder. Leave a little room for this and allow for space in case it freezes. In the winter, i leave my rtds in the trunk (New England Weather) so they are nice and chilled when i get to them. Had a bad experience with the isopure rtd's (translation - glass everywhere).
Leaving a little space in the bottle allows for room to shake the bottle when the protein has settled to the bottom. With this in mind, please have a cap that can be screwed back on, it kills me to have to try and get my thumb to cover the opening of some of these cheaply bottled rtd's only to have leakage. Please go wider mouth, with cap
My rtd's tend to be good pre and post workout supplements thus I tend to lean towrds more readily absorbable forms of whey. With this in mind, refreshing flavors seem to be what i lean towards. Preworkout rtd of choice has been lemon green tea by isopure, flavor is not the greatest but like having green tea pwo, consider a better tasting green tea flavor (very partial to arizonas diet green tea flavor). Just a thought and personal preference.
I tend to reuse some of the bottles, as an aside would also be nice to have oz or ml labels on the side. Benefits are two fold - I reuse the bottles, sometimes with other proteins, so we help with reducing waste. Secondly, the bottles I reuse have the branding of the RTD I purchased, which is visible to all around. Free marketing.
Hope this helps, just a few of my thoughts.
Cheers.