That means that there is a very high chance something Donkey Kong Universe related is in the works. 

My opinion is that it is most likely a Diddy Kong Racing title.

Why Diddy Kong Racing?

1. Retro Studios, although a long time ago now, was given experience in developing remixed tracks for Mario Kart 7 for design training purposes. This indicates that Nintendo had it in their minds that they would develop within that genre. Nintendo's closest IP to that is Diddy Kong Racing.
2. Diddy Kong Racing was popular on N64 - at the time of its release, it was one of the fastest selling games in history.
3. The Mario Kart-style genre is A LOT bigger now than it was on the N64. Look at the sales of Mario Kart DS (23M), Mario Kart Wii (36M), Mario Kart 7 (14.3M) and Mario Kart 8 (12M+ and growing).
4. There is a noticeable gap in Nintendo's lineup when it comes to racing games and the existing demand for them on Nintendo consoles; especially Mario Kart style games. Mario Kart 8 was a launch window game, but fitting another one in for later 2018 to early 2020 would be mint! (sorry, I had to! I haven't used that millennial buzzword before)!
5. David Wise is working in Austin Texas, where Retro Studios is located.

Why not another DKC title? They are most experienced in that.

Answer: I think it could be that too, but less probable.

1. No need to fill a 2D platformer gap. 2D platformers have seen an incredible resurgence in recent times. The primary platform for these is the Switch, there are TONS of them coming out, and people are gravitating toward them. Putting a DKC title on there won't be filling any gaps. Nintendo themselves already has two other new 2D platformers coming out with Kirby and Yoshi.
2. We are getting a somewhat new DKC game in 2018. While some players have already played DKC TF, the original sold to very few of the potential fans.  The release will be an original title to the majority of Switch owners. Looking at DKCR, it's clear that most potential DKC fans didn't buy the Wii U for TF; sales profoundly declined in sales from DKCR (It sold only 26%).

Don't get me wrong, I think Nintendo is considering a new DKC game for the future, my doubts are that it is due for release this year or anytime soon.

Under the assumption that Diddy Kong Racing is a reality, I would guess there is a concept and prototype for a new DKC game is already in existence.
There's potential that part of the reason for the port of TF is a bit of a business case test before Nintendo greenlights another DKC.
If DKC: TF sales projections hit certain goals - then Nintendo will hammer that green light!
Alternatively, if the game doesn't hit targets, the concept/prototype will be put on the shelf until next generation.

My estimate is that the green light number is probably 2M units this year.

David Wise working for Nintendo link: http://nintendoeverything.com/rumor-david-wise-working-on-something-at-retro-studios/

I would rather have a port of the original Donkey Kong arcade game than Diddy Kong Racing 2...

The problem with a Diddy Kong Racing sequel is that the roster is filled with RARE characters. It was essentially RARE's take on Mario Kart. Aside from Diddy Kong himself, they would need a whole new cast of new characters that have no iconic impact.

With Link, Splatoon, and Animal Crossing being added to Mario Kart, it has evolved into something like Smash where it features many Nintendo IPs rather than just Mario. With that in mind, many might see a new Diddy Kong Racing and question why they did not just add him to Mario Kart since the games are so similar. It might look like a quick cash grab with unfamiliar characters and I just do not see how they can pull it off and have it sell without RARE.

I loved DKR, but my hopes for a sequel left when RARE went to MS and took their IPs with them.

There is nothing more sad than the idea that Retro has been wasting 4 years making a racing game

I'd prefer if they just worked on the next Mario Kart instead. And if there is a Diddy Kong Racing game in the works then I really hope some random 3rd party developer is making it. Nintendo announcing that say Retro Studios next game is DKR would kill me. I know I'm being super-selfish but that's how little I care about Diddy Kong Racing.

Nintendo should resurrect F-Zero if they want to make a racer that isn't Mario Kart.

I loved Diddy Kong racing back when it came out.

You... you can't be serious O.o

EDIT: I just remembered that Nintendo announced some months ago that they would port its arcade games to the Switch. So, the original DK will eventually come. VS Super Mario Bros came out not long ago...

Anyway... not really a fan of Diddy Kong Racing. It's evident that Wise is working with Retro Studios based on his location (Texas). But I really hope that the game is something completely differen than this. Retro deserves better.

Eh, I wouldn't be that big on it. Kart 8 DX does a brilliant job as a mascot racer and Retro have been silent for four years. I'm also not sure if there'd be another DKC game coming - especially with a port of Tropical Freeze confirmed.

Personally, if it's true Wise is working at Retro, I'd hope this is either a 3D DK game - possibly something more open-world and sandbox orientated - or a new IP.

I honestly think they could be working on a 3D Donkey Kong title. That could be the only thing that would take up the past 4 years of whatever it is they've been working on and being so secretive about.

I've never really cared for Diddy Kong Racing, so I'd be a bit disappointed if this really was Retro's next project. I'd happily take another Donkey Kong Country entry instead honestly.