I have always liked the Guilder class, and i am wondering about adding one to 5E.
Perhaps the guilder could be another rogue archetype. However, there are a number of rogue abilities (sneak attack, cant, dodge, evasion, etc) that may not fit the guilder. The rogue in all guises is clearly a sneaky type, but a Guilder is not (necessarily) so.
So maybe the Guilder could be justified as its own new class, with its own paths: adventurer, trader, things like that.
I like the idea of the Guilder, and i think the 5E system could support one , but i am undecided on the best way to implement.
I would go for a full class, and with different paths. One that was suited to the Brecht tradition, another for the Khinasi traders.
Another idea would be a Cleric domain for Priests of Sera, a guilder domain that was centered around trade. The Sterling Vermin Cleric handbook has a domain of commerce, that could be adapted for BR.