I prefer borders as they was in earlier alphas, looking like wastland, but I guess water can also work as border, giving an island feel. The most important is that players get a chance to understand that the borders is getting close
Hadnt thought about adding water, though I have gone on a few unsuccessful hunts to get my hands on some of the experimental island maps from a few months ago, im really regretting not making 20 of those to use now lol. I could do water, but it will be prefabbed in and probably wont give an island effect. Water will continue to be underutilized till they fix it, hopefully next version. Mos of these new maps, like my others, i cannot seem to get water too deep, which honestly poses no problem for me, as i can and go generate an occasional 10+ deep lake if it serves some purpose on the map, but IMO they are never 100% as good as the natural ones.
Im doing this smaller map as a test run up to your map and so far the wasteland border does the job, though i made sure no other wastelend is touching the border anywhere, that would be the only tricky place to detect it for a players, as im hand making them, i will try my best to make the line clear in those spots. If we had custom biomes to spare i would use one for that purpose, but i think they are too valuable atm for that.
I have not thought much about this, I just like there to be many different biomes in mixed sizes mixed together. Overall I guess something like this would be ok:
Forest 30%
Snow 15%
Dessert 15%
Water 5% (Mostly rivers and smaller lakes)
Wasteland 15%
Burn-forest 15%
This is roughly the ratio i shoot for, and on a 16k map, it will be easy. Water we are at mercy of map, but that % is not hard to achieve naturally. Lakes more than river, but some rivers as well. Snow, forest and desert are my anchors. Wasteland i give 10-15%, then spread some out in spots tied to POIs. Burnt forrest i use more for a transitions and borders, maybe one large burned out forrest, which for 16k will be substantial.
On a 16k map, biomes could be anything from 500m to 3km. Also separating biomes with tall mountains and rivers is nice.
Theres nothing worse than a really noticeable biome change right in the middle of a nice area. I always split up biomes based on the natural features. Certain biomes also blend nicely where others look stupid, so i only blend transitions where it makes things look better or more real.
I also got permission to pack my maps with the hdhq pack included, so im really going to tweak the tree numbers and things included in prefab spawn for trees etc.