I think that Mario Kart and Super Smash bros are different situations.
In the case of Mario Kart, I don't think anything less than Mario Kart 9 would be acceptable. Mario Kart is probably Nintendo's most reliable series of console games, consistently selling to at least one in three console owners. And simply put, ports of three year old games don't sell as well as brand new games would, even if the original was on a less popular console.
For one example, look at Resident Evil 4. It was originally a GameCube exclusive, but was ported to the PS2 less than a year latter before being ported to the Wii. Heck, a PS2 port was announced before the GameCube version was even released. Considering how the GameCube sold about an eighth as well as the PS2 and a fifth as well as the Wii, you'd think RE4 would sell relatively huge amounts on those consoles, especially since the GameCube was dying off years before the PS2 would. And the PS2 version had extras. But even the PS2 version only outsold the GameCube counterpart by about two to one. The same thing happened with Dreamcast games like Sonic Adventure to the GameCube.
Now, Mario Kart 9 doesn't have to be a revolutionary makeover of the series. After all, the gap in hardware capabilities and years between installments is smaller than usual. The graphics can be fairly similar to those of MK8, there can still be 12 racers at once, and so on. But it still needs new elements besides a few new characters and tracks. It could be something comparable to Double Dash's two racers per kart and character items, the Bikes and Tricks of Mario Kart Wii, or the gliding and antigravity of MK7 and MK8, etc.
My hopes are for more multiplayer options, both locally and online. Being able to play with 8 or 12 people locally using multiple Switches would be cool, and I fully expect online functionality to only increase after MK8. Being able to hold multiple items and switch between them could be a new gimmick that adds depth without being too complicated.
As for Super Smash Bros, I'm going to say something heretical. I think we can get a new game in the series without Masahiro Sakurai being in charge.
Hear me out.
Sakurai has repeatedly shown disinterest in making new SSB games. As far back as Melee, he was working under the assumption that he'd never make another entry again. And with SSB4, it was largely a collaboration with Bandai Namco's fighting game developers. If Nintendo wanted to start working on a new game, let's say in time for the 2019 holidays (a five year gap), one of Nintendo's studios (HAL Laboratory and Nintendo EPD both have series veterans) could collaborate with Bandai Namco.
Sakurai would supervise, but he would mostly be working on his new pet project. Maybe he'll revive another dormant franchise. Mole Mania Uprising?
Love and tolerate.
