More specifically, it's one product/brand name as opposed to another. Ratchet & Clank is its own IP and Resistance is another.
Anyway, I actually like it quite a bit when they play with the formula a bit.
SAC beats out more than a few R&C games for favorite. Probably only behind ACiT, and the original trilogy, really. I really liked SAC, believe it or not. It was a really interesting take on the series and it was just fun. Lots of minigames and the Qwark sections were crazy fun. Especially the opera.
Despite all the complaints, I thoroughly enjoyed A40. It was certainly different, but it felt like an R&C game to me. Just with a wonky camera and three friends. It was fun and I think it should get more respect as an R&C game. Maybe not as much as, say, ACiT, but it's worthy of the R&C branding.
FFA is the only "experimental" R&C I didn't really care for. It was worth $20 for sure, but the gameplay it presented was more suitable for being a section of a bigger game rather than being its own thing. While I didn't like that being the whole game, I wouldn't mind seeing some MOBA sections in future R&C games. Yunno, present them the same way they did the Galactic Ranger skirmishes in R&C3.
All in all, I dig it when the formula is played with. Keeps things fresh. Though I understand why someone might prefer that they try out a new IP rather than making an entry in a preexisting series so different.
they'd have to change it up so much gameplay-wise that it might as well not be in the R&C series for the same reasons that people said earlier in this thread that A4O and FFA should have been part of a new IP…
I also wholly disagree with this. Adding more things doesn't mean changing the way the game is played. It means adding more things.