I don't know about anyone else but warframe slots are a royal pain in the ass to obtain, especially since i am unable to pay online and there are, to my knowledge, no gift cards that i can purchase in England that are valid with Warframe which is why I think it would be a not-bad idea to reward players with a frame slot every 5 Mastery or so (Yes I am bearing in mind the maximum mastery so far is 18 but that will rise as more content is added) since this somewhat remedies the pain that i endure seeing every single frame (including the Frost Prime) ready to be claimed but unable to do so.
I chose to say every 5 mastery because that requires quite a lot of effort meaing you have to work hard for the free slots and also gives players more incentive to play around with a wider range of weapons and frames and it is a challenging milestone which is limited by max possible mastery level thus the developers aren't losing out on too much money and retains their theme of 'not pay to win, pay for your laziness to get shit quicker' (plus by getting slots one is able to acheive higher mastery rank)
It would probably be more sensible for me to say you get a single free weapon slot every 5 or 4 mastery since they are valued less and weapons can be sold and replaced easier than frames (I feel its nice to keep all of them as unlike weapons they all have different uses and playstyles that could help in many different situations) however as my problem mentioned before proves frame slots are pretty damn annoying in my and probably other peoples case.
I could even say Mastery 5 = weapon slot, Mastery 10 = Warframe Slot and so on in that fashion. Just an idea :3
TL;DR Version: I'm lazy and poor and need moar free shit!