[WARNING: Very long post]
So, after reading through Dan's post of whether or not IS could implement a trading system into the game I gave my thoughts on the matter. However, I began seriously thinking of some stipulations that IS could put in place to discourage abuse of such a system. Some of these I felt were obvious, some were revised from what I had posted on that question's page, and the others are based on the comments made by ArgentSable and DriftingInRhythm. I will list the requirements in a numbered list, followed by my reasoning as to why each requirement is necessary in separate paragraphs below the list.
(1) Maximum of one trade per week.
(2) You can only trade with one particular person once per month.
(3) You can only receive a particular unit once per month through trading.
(4) A new, non-purchasable currency must be introduced for trade, which can only be earned through difficult in-game challenges (such as Squad Assault, GHB Elite quests, etc.).
(5) Both accounts in the trading session must be at least one month old.
(6) A total playtime of 4 hours (per week) is required on both accounts before conducting the trade.
(7) For a unit to be eligible for trading, it cannot have any inherited skills and/or merges.
(8) Traded units must be of the same rarity.
(9) No limited units (from Tempest Trails and GHB) are allowed for trade.
These stipulations are designed to prevent people from creating unlimited, new accounts to transfer bad units over for good units, while simultaneously preventing veterans from boosting a new account (or their own account), without heavily limiting the amount of trades a person can do. One trade per week I feel is generous without being too limiting (as a trade per day is just too crazy), but I'd imagine that a limiting resource would even prevent many from trading once per week anyway.
In particular, conditions (2), (3), (5) and (6) are there to prevent making a permanent, throw-away second (or third, or fourth, etc.) account to boost the primary account. Basically, the restriction is there to (hopefully) make it more trouble than it is worth to trade for the people with multiple FEH accounts. The one character per month makes it so that all merge projects would take at least 10 months to complete via trading only while not totally screwing them over if they get super unlucky, which I feel encourages players to continue rolling on banners to speed up the process. Most people can achieve the 4-hour requirement just by playing normally; 35 minutes a day means that you can fulfill the requirement in 7 days, which is not unreasonable (such as commute time to school, free time during work, etc). But for the people with multiple accounts, that's an extra 4-12 hours, which (while admittedly not much) at least puts more restriction for those players--again, making it more trouble than it's worth.
Conditions (7) and (8) are there to prevent people from abusing the feature to send premium fodder or highly merged units to other accounts. (For example, giving your 3-Star Marth Even Spd Wave on one account, then trading him to another account that doesn't want to give up any 5-Star units.)
[Condition (9) is mainly here to preserve the rarity of limited units, as IS doesn't like to give them out very often, and I don't think that they would be willing to allow anyone to +10 them just by trading. :/ ]
Of course, I'm pretty sure that I am still missing a bunch of possible exploits, but I just wanted to write down something to articulate my thoughts and encourage discussion (if anyone is actually willing to read through all of this, lol). I really would like trading to be implemented into the game in some capacity; perhaps a post like Dan's or mine would allow people to come up with some solutions to these problems without being too over-restrictive, and encourage them to send a feedback to Nintendo about the feature with some of the restrictions that we though of. At its very core, the trading system should be a way for one to obtain his or her most favorite units in the game without discouraging them from spending on new banners.
Who knows? If we bug them enough, we might just be able to trade units one day!
As a side note, ArgentSable also brings up the idea of a unit shop in Dan's post, which I really like and can be implemented instead of the trading feature, or in conjunction to it.