vvv Summary: Different Uses for the void. vvv
One thing there is to consider is that there could be multiple uses for traversing the void. I personally love the rarity of the portals that hop from one island to the other due to making them more unique and possibly defended by another faction or entity. Though, now that we're in the suggestion thread, I shall suggest possibly using the void in result of not only doing island hopping but also, in some cases, making your way to different dimensions. This "void" dimension would possibly make more sense if it was used in more of a diverse way, looking at it as more of a half dimension, a split between two or more realms. It's interesting to see when you use these portals to not just fold space and time in half then just to walk from one island to another like a standard teleporter or warp gate, but merely used to enter an entire different dimension as a path way. This "dimension" instantly gets a lot more concerning for its existence and properties.
vvv Summary: Different functionalities for the different uses of the void. vvv
The reason I thought of using this "void" dimension for different purposes is due to the point you, Stroam, made of if we should be able to "summon" these portals along with other game changing features. If this "pocket dimension", "split dimension", "half dimension", etc. is used for passage ways to a multitude of locations being either the same dimension or another, then some portals would make sense to "summon" while others, like the ones that just transport you from one island to another, isn't. This would be able to maintain the rarity and seriousness of more concrete and permanent portals while using lesser portals for lesser counterparts, having both of these two mechanics using the same system. These different variations of transportation used in the void could have different stability, functions, and encounters.
vvv Summary: Overworld void corruption and random void rift encounters. vvv
Also, I believe void like things could make for some really interesting fantasy end game tiers and weird lore if lore were to be added in the game. Expanding on the point I've made about how some portal's wards get destroyed, due to this "void" dimension possibly being more of a 0.5 dimension than a complete one, you could have rifts and tares in the fabric of space which lets out this corruption and taint into various worlds because of its natural instability. There could also be a feature to create random encounters that make you go; "WHAT IS THAT?" and either force you to retreat or bunker down. I personally love when a game adds random events that could happen out of the blue which makes you a lot more cautious with the environment. It actually gives you reason to build shelter more than the fact of comfort. I believe some players would possible give up on building a base or house on every island or so besides small two block high walls or something to prevent monsters from coming in. Though, random encounters like a void monstrosity ripping a hole into your dimension to say "what's up", or possibly more of something like a storm instead of a monster emerging from an unprotected rift to the void, will gives players a lot more of a reason to build at least some kind of fleshed out bunker. Also, one thing to make important is that I see that every hostile void creature would be aggressive to anything that isn't of void descendant.
vvv Summary: Attempted justification of making this particular aspect of the game so expansive. vvv
I know I'm putting probably way too much effort into looking at this one topic of this mod compared to everything else, I just can see a lot of potential that can be tapped into with just this one aspect. From my experience with fantasy and magic, the void tends to be absolute end game territory. Fantasy concepts usually don't get too in-depth into the nature of the void, but almost always show how expansive and powerful it is, especially how dangerous and corrupting it is. In result, from what I know, this dimension and island transporting concept is very lightly done so far and isn't that deeply thought through. Yet, I believe if TFC2 is going to be fantasy related with all of these planned features, while having these portals and this void half dimension placeholder already in the game, it would be a very good idea to expand this aspect of the mod to add such more alien and unique gameplay. Most people know generally most of the creatures to encounter in a fantasy world that's like what's planned for TFC2, but nobody quite knows what kind of horrors that could reside deep within the void, especially the aspects that are strong enough in which they have self will and consciousness to attempt hunting you or anything specific down.
In my head, I have a mixture of these following mods:
Thaumcraft
Astral Sorcery
Ars Magica 2
Dimensional Doors
These are not due to their items, creation, and research focuses, but instead more exploration, vibes, and discovery focuses. I love the vibe and reality in Minecraft they attempt to create which would possibly work great with expanding this aspect if not others inside this mod.
Though, I believe I've finally drained everything that I can think of with this aspect. Again, sorry for the ridiculous amount of text, I tend to put too much effort into my work when I get my hands onto anything. I loved TFC1 and love seeing the progress of TFC2 so I wish the best for this mod. Though, like before, I thank you for the ones who do read through my massive mess. I really wish I was able to get into programming enough so I can actually start making my ideas myself or helping/working with others. That's one of my later goals in life to do, I already know the basics of several languages, I just need to chisel out my path or receive help till everything becomes clear.