I think that -1 interest is extremely good. I expect a playthrough as Gotland will go slowly at first (I assume Gotland is independant in some starts, no? You can literally start on any day between the start and end dates of the game). Once you do get a couple of idea groups unlocked, if you haven't expanded madly, you can use the -1% interests (esp. coupled with Economic ideas) to fund a gigantic army of mercenaries to take on enemies twice your size. Or more. Just a pro tip: Plan this strategy ahead of time, as you want to have the mercenaries win the first few battles of the war, after which you can start disbanding them.
Another strategy is to use the low-interest loan to fund enormous building projects... If you stack -3% or even -4% interest it'll essentially always be worth taking loans to build buildings, assuming your inflation can be kept under control (which can also be made easier with ideas.)
The minimum interest rate is supposed to be 0.25% (4% is standard). Last I checked, there was a bug that made -3% interest from ideas set your interest rate to 0%, though, which was kinda funny but totally broken. I assume they've fixed it or will soon. Either way, 0.25% interest is almost free money, excepting the inflation. If you're lucky, you'll get an event allowing much higher loans, which means less inflation per ducat. Again assuming you'll eventually be able to repay the money and keep inflation down, this means a moderately sized nation can build every building they have monarch power for, or make overwhelming first strike attacks against large neighbours, destroying their armies with enormous numbers of mercs who can be disbanded when no longer needed.
I definately think it's okay for this idea to be far down the line. It's only 3 full idea groups... Delayed somewhat if you spend admin points on econ and admin and innovative, but assuming your Gotland isn't a major world empire by then, the options it opens up are huge.
