(Apologies for how long this is, but it kept growing as I was writing and I tried to re-arrange it in a way that wasn't too scattered.)
An art book is something I always ask games to make, especially ones where I don't need to worry about it being censored by a western release. Most high quality books can be MSRP'd for about $30-40 brand new (look to Udon brand for an example, etc.).
If you can manufacture "prize figure" type items through a Japanese company at enough of a quantity to sell them for around $25-35 like they typically go for now with a few exceptions (some of them hit $50 depending on who makes them), I would definitely buy certain characters of those.
As for things such as mugs, hats, tote bags, etc., they do not thrill me and I would likely not pay for those. T-shirts perhaps, depending on what is on them. Simple logo shirts usually don't appeal to me, but if you could get prints with your particular favorite characters on them or your favorite skins (robutts art preferable), then you have my attention. If you end up with some particularly interesting kaiju as well, purchases may be on the table. If you really want to compete, however, it will have to come in at $20 or less(pre-ship) to be feasible for many in the market. There are many, MANY, shirt sites who sell at 20 and below and they have no qualms with infringing on properties to sell them. Just check out things like teefury, etc. and you can clearly see many properties, mainstream or otherwise, with merch. If official is not able to compete for the prices, then there's no incentive for the mass market to go with official. No one cares about the status of it being "legit" rather than a rare few. I understand in a perfect world, you'd say "The fans, whom really care, will do the right thing." but the fact is, most people do not have that noble bone in their body. If you want to succeed, you have to find a way to compete.
Also, whatever else you make, please take into account and try to make it affordable and not overly bloated on price. Have differing versions at the very least. For me as an example, I'm not paying $100 for a pillow, I'll tell you that up front. I will however buy multiple cheaper things that add up to $100 but a single item is typically not worth that huge of an investment. Some will pay reasonable prices for a few things that add up, and others may only be able to afford smaller things period. Please cater to the normal players too and not just the super whales. It'll pay off in the long run.
ps: To clarify about waifu stuff in particular: Always cool & welcome, but often very overwhelmingly expensive arbitrarily. If, again, you can bring it down to prize figure levels or near there, $25-35, then we're in business in regards to figures. just do not jack up the prices simply because it has a waifu. I've seen it done and it's a disgusting practice. The only exception would be if it needs to be a few bucks extra because it supports the artist or something, but even then it needs to be clear across the board and not arbitrarily applied to certain items, you know? An extra couple bucks on a t-shirt purchase that helps support robutt or another artist directly would be reasonable.
I realize this has dragged on for a while, my apologies. I just want to see you succeed because there are very, VERY few people working to make games that are telling the "woke" mob and such to butt out and making the games they actually want to make. I want to see it continue and become widespread again like the old days. Thanks for reading.
EDIT: Oh, also please include hoodies (if you decide to sell them) with the design printed on the front as well and not only ones with the design on the back. Some people, like my sister, loathe it being on the back because they want to see the design they bought themselves and not just other people.