If you want to be able to claim all
the credit then nothing less than 100% will suffice, breaking this down you have:
Interface: window furniture, fonts and icons (including templates).
Transport infrastructure: roads, ways, bridges, tunnels, stops, docks, airports, stations and infrastructure
Vehicles: trains, buses, lorries, aeroplanes and boats.
Industry: factories, farms and power plants.
Buildings: Monuments, attractions and city buildings.
Landscape: Ground textures, lightmaps, transition images, trees, marker, grid, foundations (natural and manmade) and water.
Now, completely replacing all vehicles is in itself a big step. It can effectively localise a pakset to a different country, you can go for a simple or complex range of vehicles, focus on one form of transport etc. If these effectively fit in with another pakset then you can think of this as a vehicle addon/replacement.
Moving on to transport infrastructure, industry, building and landscape, these can all add to the feel of a distinctive pakset. Some graphics few paksets try to alter - lightmaps and transition textures aren't immediately visible to the player so may be shared. Interface graphics are rarely changed as most paksets concentrate on the playing world.
Why is it so important for a pakset is 100% your own work? Even paksets which may otherwise be completely different may share some graphics (some buildings between pak64/128, certainly in the past for example). Unless there's a good reason most paksets of the same size will reuse marker and grid images (maybe with minor colour changes).
Creating an environment which looks good and is interesting and challenging to play is the most important aspect of pakset creation in my opinion, how much of that is your own creation is neither here nor there. People will acknowledge the work you have put in, however much that is
Just seen your comment elsewhere:
Isn't people working on too many project? Maybe joint project might speed up things.
There may be some truth in this, pak64 and pak128 have had many contributors in their time....