Ok, well you have asked people to speak up, so I hope you will take this in the manner in which it is intended.
You say you have yet to take part in any GB's. Might I suggest that you take part in a few before staring your own. Running a GB seems easy, start a thread and away you go. But it is far from it and it does really help if you get a good feel got GB first. All to often we get GB's started on a whim without any thought as to what else is going on, or without any planning, simply because they want to build this or that kit. And while we have nothing against new members, of course we try and welcome all, like with any group, it does help to get yourself known. Many of us were on the site for a couple for years before starting our first GB.
As for your subject, its a great idea, but I think to limited. You are right in that we don't see many float planes, and there may be a reason for that, and its not simply that there is no GB for one. Some subjects can be very narrow. take the Bf 109 GB, there are probably more 109 kits that float plane kits of all kinds, so the 109 works with a GB for itself. But floatplanes are less common so a wider choice might be better. rather than just single engine, how about all float planes from all era's.
And finally, I am a huge GB fan as I am sure you can tell. But over the last couple of years the forum has gone from being diverse, with a mix of both GB's and WiP threads to being very GB heavy. The large number of GB's is, in my view, not only detrimental to the rest of the forum but to the GB's as well. Surely quality is better than quantity. The large number of GB's means that many don't enter because they show restraint and decide they are already in to many, or they over commit and simply don't build for many which spoil what could be a good GB.
Fortunately on this forum we are given the freedom to run GB's by ourselves, but we should show some responsibility in this. We do have a lot og GB's ending at the end of the year, but we have 5 or 6 starting on 1st Jan, and yes, I know, one of those is mine, so maybe I am being a hypocrite. But at the moment, we are looking at getting the GB numbers down going into next year which I think will be a good think. And for the sack of your GB idea, I would suggest putting this on the back burner until later next year. take part in a couple of GB's, hopefully the numbers will thin out a bit, and you may be able to host a successful GB. And if its opened up to all float planes, I will certainly be in.