Hong Kong doesn't need to be bigger. Guangdong, at minimum, needs to remain in ROC hands, but that's going to be a lot harder than holding Hainan.
You'd need a pre 1931 PoD, preferably a pre 21 Demands PoD, to allow for any sort of Japanese cultural presence to survive into modern times.
Not just that (in the case of the latter in terms of Japanese cultural influence - which did happen to some degree in the Qing dynasty) - in order to have, IMO, any ROC to live up to its name in an ATL (from a Western POV, hence addressing the former), the ROC would actually need to have more than just islands near the Mainland. In that case, if one of the 1940s lines holds, that would be great news for the ROC - and hence the eventual United Provinces of China.
Before we go any further, let's have a look at this map, which shows the notional ROC claims on the Mainland vs. the PRC's administrative divisions including the claimed Taiwan Province.
<
https://en.wikipedia.org/wiki/File:ROC_PRC_comparison_eng.jpg>
This provides a good idea of where things are in relation to each other. On the map, PRC placenames are in Hàny
ǔ Pīnyīn Fāng'àn (aka Pinyin), while ROC placenames are in "traditional"
Postal romanization.
Now, first off, let's start off at a later stage, that of trying to hang onto as much Mainland territory as possible in the South - so around the time of the flight towards Taiwan and all that. Now, the ROC claimed that while all provinces were lost, the following provinces had at least some territorial hold by the KMT/GMD:
*Taiwan (Táiwān)
*Fuchien (Fújiàn); known as Fukien on the map
*Kiangsu (Jiāngsū)
*Chekiang (Zhèjiāng)
*Szechwan (Sìchuān)
*Hsikang (Xīkāng), known as Sikang on the map
*Kwangtung or Canton Province (Gu
ǎngdōng)
*Hainan SAR (H
ǎinán)
*Yunnan (Yúnnán) - which IOTL had the notoriety of having its capital-in-exile at Bangkok
*Sinkiang (Xīnjiāng)
There were also two additional special municipalities, Guangzhou and Chongqing, that were claimed but for obvious reasons, well, -
Now, as I see it, anything from Guangdong up is fair game; anything east to that would be problematic (Fujian is optional to have at least some retention of Mainland territory, at least in the areas near Quanzhou and Xiamen (Amoy), but the rest would remain as island possessions, though at the bare minimum only those areas of Fujian retained IOTL by Taiwan would count) and would probably, in this first scenario, remain island territories (Fujian, Chekiang) or titular (Kiangsu). Xinjiang itself can also be sacrificed as it's too far away from the rest of the ROC-held territory and would hence also be titular unless the Ma clique holds firm in Tsinghai (Qīngh
ǎi), in which case not only Xinjiang but also Tibet could be counted in. Which leaves the core remaining ROC territory (Guangdong, Hainan, Yunnan, Szechwan, Hsikang), which has a giant hole separating Guangdong from the others. That is unacceptable and needs to be modified. First off, Guangxi (Kwangsi, Gu
ǎngxī), Guangdong, and Hainan are somewhat closely related to each other, to the point where they shared
the same viceroyalty under the Qing dynasty. In particular, along with HK and Macao, Guangdong and Guangxi are closely related culturally as part of a general
Lingnan culture - even in the case of Guangxi with the presence of the Zhuang people. That helps close some of the gap, but not completely because Southwestern China is very mountainous and thus there is no form of good connection with Yunnan, let alone any connection between Guangzhou and Chongqing without having to cross PRC territory. To be sure of such connections, Kweichow (Guìzhōu) would be needed to complete the ensemble. This minimum scenario (minus mainland Fujian, Tibet, Qinghai, and Xinjiang) would probably be the closest both to a Taiwanese model of economic development as well as the post-Mao Chinese economic reform, except the democratic transition would include a period between CKS's authoritarian military dictatorship and the eventual blossoming of full democracy as modern OTL Taiwanese would know it (and hence the United Provinces of China), as well as many others, where the ROC is basically a mega-Singapore on steroids or Mexico during the latter days of the PRI's classic period. Yet Hong Kong would play a vital role here as breathing space for all sorts of opinions that would not be tolerated in the ROC, such as the predominant usage of Cantonese instead of the ROC's insistence on Mandarin (for example), which will eventually prove to be somewhat influential on the Mainland.
There is yet another scenario, which thus fills in a certain region that the ROC already recognized back in the day:
Combine the dark and medium red areas together, and you get the classic definition of South China (華南, Huánán) as the later pre-1949 ROC understood it. In this case, take the scenario above and add Hunan (Húnán), Kiangsi (Jiāngxī), and mainland Fujian - and then you roughly get what the frontline eventually was before 1949 changed everything. Chekiang would remain a bunch of islands here, Qinghai is uncertain and Xinjiang could still be written off as a loss, but at least it would be more retained territory by the ROC.
The ultimate, which I think would be ASB but would work better if a longer Qing dynasty is substituted for the ROC pre-WW2 (say, for example, the Qing dynasty basically "pulls a Meiji" - literally, in that an overcentralized system becomes even more overcentralized and ultimately descends into a system more fascist than Shôwa Japan, but at least we get our industrialization in the face of everyone else who prefers a weak China - and hence because of its pretty more-than-overt sympathy to the Nazis gets broken up and occupied by the Allies, creating a Chinese Soviet Republic in the North and a United Autonomous Provinces of China in the South) but would be the ultimate for a surviving ROC in this case since that would mean that Nanjing and Shanghai would at least be "safe", is going for the traditional boundary line separating northern and southern China - the
Qinling Huaihe Line, which means Jiangsu and Anhui get sliced in half, small pieces of Henan and Sh
ǎnxī remains with the South, while a small piece of Hubei remains with the North, and Szechwan is intact within the South (while Gansu is intact within the North). Basically, imagine that east and northeast of Sichuan/Szechwan the light red area remains part of the ROC.