Why not use the Month system ?? Log in every day for a week will give you a prize, potato, Ayatan, Riven or other item in that same group, would be random, but above the MR10 the player can choose the prize of the first week,
Logging in for the entire month without missing a day, you win a coin, which can be exchanged for mods, visual items, weapons and everything the current system has, and more, exclusive items from that "Login Store"
Players who have accumulated many days can complain about this system, but they will be able to accumulate many more coins than new players who have a lot more things to buy, not to mention that we would have exclusive packages from the "Login Store"
These packs would be thematic skins of warframes and weapons that would come in events, "Halloween, Christmas, etc etc" in case it would be the secondary way to get these items without being playing possible events,
That is, players with more days would spend less coins, and would have the ability to get these event packages without much effort,
but the days system would still work, ie, it takes 5 days to get a gun or mod that you have not picked up and 7 days to get a coin, you would receive the item in the 5 days and could spend the money with something else after 7 days
But this store should receive items regularly, to avoid players getting everything and accumulating currency for lack of something to buy, obviously it is preferable that it be visual items, but can be weapons for a limited time