A lot of work, potentially for a long time. Maybe Dialog will hire you, or at least pay you. (or maybe they'll stop making the chip. Or go out of business. Or be acquired by some other company.)
So let's say you do the port, and you think it's "finished." At a minimum, you still need to:
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Watch for user complaints indicative of bugs in your code. Or features you didn't implement (maybe features you didn't plan to implement.) Some significant number of these are likely to be "stupid questions" not specific to your core or the dialog chip
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Watch for changes from Dialog in their presumably-semi-proprietary Radio code (assuming you already figured out how to include and support that in your Arduino core.) Or if their SDK changes. (Atmel/Microchip is on something like the 4th backward-incompatible iteration of their ARM SDK. SAM Arduinos (Due) uses Rev1. SAMD support (Zero, etc) is "bare metal", but is now using an "outdated" version of the CMSIS files.) Make fixes and changes as appropriate. Support new protocols as they come along and reach a certain market density. (IPv6? The latest encryption?)
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Be prepared to need to add "variants" if the chip becomes popular enough for third parties to start building boards.
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Keep an eye on your source code repository provider. Arduino used to be on "Google Code", but it went away. Github has been bought by Microsoft...
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Watch for changes from Arduino in terms of what is included in their core and APIs, as well as how your cores is installed and updated. You can choose to ignore changes and hope your users are OK with "the old way", I guess. Or you can update aggressively and annoy the users who DID like the old way. This means watching whatever official Arduino core you are closest to as patches go in, and figuring out whether those patches are relevant to your core.
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Watch for libraries that become popular that aren't immediately compatible with your core. If they're popular enough, port them. Have fun if they're fundamentally incompatible (WS2812 "neopixel" is a good example of something that is moderately difficult to port between cores.)
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Maybe things go great, and you collect a bunch of other people "helping" to "add features" and "fix bugs." Be prepared to spend some time evaluating their contributions and saying "no" a lot (but "nicely"!)
You might also worry that Dialog decides that they really like having Arduino Support for their chips, and goes off and hires someone else to do it "better" than your core (or does it internally.) (I don't know exactly what happened with STM32 and Roger Clark, but it doesn't look much like I would have liked it to go.)