Yes they can, greatly. If anything more on the technical end of things, since the engine has to support all the factors those modes implement. But think about it simply; randomized level layouts effect the design structure limitations, especially if they fancy reusing main game rooms in the mix. Or maybe they have to for budget and time limits. 3D work takes a long time to do, and MANY people. Level design, is a core component of the game. It effects both main game and randomized mode.
That is just one of many factors, and a really simplified explanation of it. It could be as specific as "Well he randomized mode needs level data to now have this, and remove this, now we have to rewrite this segment of the code, change this, figure out how to accomplish this without what we removed, and reformat all the files." What if a feature from the main game just cant be fixed to work now as a result? It happens all too often in game development.
I think they have good intentions, but are getting too starry eyed. They're spreading their core concept in directions that could cause some big issues. This is the kind of behavior that leads to development hell, or big cuts in content.
I don't know how else to say it. You have to look at it through the design and production lens to really understand.
I am a software developer (not a game developer, though), and you still haven't convinced me. And I'm sure you wouldn't be able to convince Iga and Inti Creates either, since there's a reason they chose to tackle this new mode.
I mean, yes, it's obvious that creating this new mode will take resources, but saying that this is a bad move because "I just want them to create the best possible Igavania" is something pretty selfish (IMO). What if for other people "the best possible Igavania" would have a mode like this or just several extra modes? You know, all previous recent Igavanias have had extra modes. People actually
liked those extra modes when they were introduced, even if they didn't think they needed them beforehand.
But let's return to the whole "this mode will seal the fate of the main game's design" issue... The new requirement states that there should be a mode in which a dungeon is automatically generated, and let's assume that I am assigned with the task to develop this part of the game...
Since this is an extra mode, I know that the main game should not be affected or altered at all if possible just to make my life easier. Then, alright, the first thing that I am going to do is search for all existing assets that will work in this mode. I will choose the ones that have the right dimensions, textures, colors, attack patterns (talking about enemies), etc. for the theme (type of level) that my dungeon will be based on (maybe even multiple themes). I will then see if that's enough to create a satisfying dungeon for each theme. If not, I will see which existing assets can be easily modified to create new ones that will work with my mode (e.g. the ones that would only need a color swap to work), and if that's still not enough, I will ask them to create newer ones as a last resource.
After considering that the assets I have are enough to create satisfying dungeons with my theme or themes, I will proceed to program the procedure that generates my dungeons automatically, and then test it, remove or modify (it it's easy) assets that didn't work as intended, add ones that I think are missing now (if any), modify enemy parameters (like their HP, ATK, DEF, etc.) for my mode, clear bugs, etc.
Now, tell me, how would that affect the way the main game's castle is designed? Do you see what I did here? I created my procedure (or algorithm) to work with existing assets. Since my mode is not the friggin' center of the Universe, I did not ask them to create all assets so they would work with a preexisting algorithm. And they would be stupid as hell to use a modified version of my procedure (with its obvious limitations) to create the main game's castle.
Furthermore, more budget does not mean a better castle. Designing a castle is art, and with that comes a limit of creativity. More money will not necessarily make a better castle, especially if it's already enough to fulfill the artist's most ambitious vision. That is why they probably decided to promise more modes instead of making the castle grow bigger with every dollar invested into it.