With only three 747’s, wouldn’t it be great to add another one? However, I know that the 747SR has few customers. Japan Airways, All Nippon Airways, Qatar, and Evergreen International Airlines, I think, are the only customers.
In general, I would think there’s no need. The -100, -200, and -300 SR models are all nearly visual counterparts so it wouldn’t offer much variety to the sim. One potential argument could be the SUD model but that could be accomplished by making a KLM 747-200 with the SUD.
Though not exactly a 747SR by naming, the only justifiable addition regarding short-range optimized 747s IMO is the 747-400D (Has no winglets compared to the regular -400) but it doesn’t offer much livery variety anyways.