So, I own a lot of the D&D content, but I tend to avoid the content that blends other products outside of the D&D realm.   Like Magic the Gathering, Rick & Morty,  Penny Arcade, etc.

While I've never played Magic the Gathering and don't watch a lot of TV (Rick & Morty), I did play Neverwinter a bit when Acquisitions Incorporated was released way back when.   I wasn't a huge fan of it, but it was more content.

The question I have is are these more of just a gimmick?  Do they alter the gameplay (change some rules) or does it just add content to the same existing gameplay? 

If it just adds content, maybe they are okay and worth getting (though Rick & Morty seems like even the content might be very gimmicky)

I ask as I was looking at the new Magic the gathering book coming out and was considering purchasing it, but I don't want to spend the money and then have buyer's remorse.

Depends on the specific product.  Guildmaster's Guide to Ravnica is an absolute tire fire, utter garbage that is banned forever at any table I run...but the Theros book is actually a pretty well done redux of Greek hero Myth-style gameplay to fit the D&D mold.  Yes, the list of gods is too long and they hadda do a few weird things to shoehorn the Magic: the Gathering in there, but if I didn't know it was a crappy M:tG set redux, Theros would've been a quite useful addition to the game.  It still is, just with a bit of tarnish for people who hate Magic.

I don't own the Rick and Morty book and I never will, but Acquisitions Incorporated is not actually a bad purchase at all.  It's quite specific to its target genre - if you're not a fan of office comedies, Acq Inc. won't do a thing for you - but the book actually finds some neat ways to make "Office Comedy" fit the world of D&D, and a number of the things they do in that book actually make a lot of sense.  There's a few new spells that feel like they should've been there all along, deceptive grifter-style stuff that makes you wonder why nobody invented that kind of magic before once you read it.  If you like that kind of story, or even just want some darkly comedic pointers on how to incorporate overbearing East India Trading Company-style tyrannical business nonsense into your game, Acq. Inc might be worth a look.

Each one of these books does introduce new rules - Guilds for Ravnica, Piety for Theros, Jobs for Acq Inc. - but they tend to be overlays on the original rules.  Additional things to use if you like, rather than irrevocable alterations of existing rules.  Up to you whether they're worth investing in, though from the sound of it I'd trend no.  For Strixhaven, definitely wait until it releases and we know whether it's actually-not-bad like Theros or breathtakingly awful like Ravnica.

It depends a bit. The Penny Arcade sourcebook is based on an actual campaign, after all - one that might deviate a bit from the norm, I guess, but is there even such a thing as "the norm" in D&D? The M:tG books, likewise, are fine. They're not mixing in M:tG mechanics or anything like that, they're simply D&D setting books based on existing lore from another game and lore is lore. Doesn't matter if it was written first for D&D, for another game, a book, or another medium - it can all work for D&D. I have the Ravnica book, but decided against the Theros one; basically because the city sprawl with the guilds was useful for me and the Greek mythology theme seemed less so. It's not really different from some people liking either Eberron or Wildemount, but not necessarily both.