I'd agree to a new series. Ratchet and Clank Future: Tools of Destruction is one of the better games to kick off the game's campaign on the PlayStation 3 in its early years. Whether Insomniac wants to drive the Ratchet and Clank vehicle anymore, I'm not sure though. Into the Nexus seems to be more like a farewell than a "More to come!" message from the development team. They've currently got their work cut out for them with Sunset Overdrive for the Xbox One. Where they'll go from there could very well be backing up Microsoft instead of playing a key role in PlayStation 4 support.
I can imagine that Insomniac will keep making games for Ratchet as they did say last year that they had "no intention in stopping" with Ratchet and Clank. Which is great to hear. If Into the Nexus proved anything, its that the devs can still make a great game.
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Don't forget though, SPOILER ALERT, that when Ratchet walks away from the Dimensionator at the end, Clank actually grabs it before following him.
That to me almost hints at another Ratchet game in the future. But who knows when that'll be!