Please go through this paper, you will get some ideas about fusion techniques and best fusion methods.

https://www.isprs-ann-photogramm-remote-sens-spatial-inf-sci.net/II-8/101/2014/isprsannals-II-8-101-2014.pdf

I suggest you see the following website: http://openremotesensing.net in this website, you have access to various MatLab codes, data and distinguished papers in the remote sensing community including fusion, classification, spectral unmixing, registration, etc.

Below is a comprehensive research about fusion techniques. I think this study will give an idea:

Article Determining the optimum image fusion method for better inter...

Take a look at the text and papers given in the links below; perhaps they might

be of interest to you (it is an answer I gave to a previous questions):

Attached are links to papers I published related to this topic that may be of interest to you (they are relatively old papers on work I did 2 to 3 decades ago).  It will show you an example of kind of 'pushing' the limits of merging very different spatial resolution image data sets.  Keep in mind that usually the spectral information if what the lower resolution data set contributes and spatial resolution (or detail) is what the higher resolution image data set contributes.  Also, I have a paper that discusses how merging multi-resolution image data sets together do better is they cover the similar spectral range to minimize edge reversals.   1991 Paper Comparison of Three Different Methods to Merge Multiresolution and Multispectral Data: Landsat TM and SPOT Panchromatic https://www.researchgate.net/publication/200458993_Comparison_of_Three_Different_Methods_to_Merge_Multiresolution_and_Multispectral_Data_Landsat_TM_and_SPOT_Panchromatic 1986 Paper Digital merging of Landsat TM and digitized NHAP data for 1: 24,000-scale image mapping((National High Altitude Program)) https://www.researchgate.net/publication/240321338_Digital_merging_of_Landsat_TM_and_digitized_NHAP_data_for_1_24000-scale_image_mappingNational_High_Altitude_Program Pat Article Comparison of Three Different Methods to Merge Multiresoluti... Article Digital merging of Landsat TM and digitized NHAP data for 1:...