About this rule:

Examples of things that are allowed:
A remake of your (or someone else's) game for a different platform, but with new code and assets.

1- Can I make a fan game made with new/original assets? (not really a remake in fact, but not my own IP)

2- Can I reuse assets from an older game I made on a different platform and remake it on game boy? (100% new code, many reused assets, my own IP)

I feel it should be good to clarify some of these gray areas :) Thanks

1- Can I make a fan game made with new/original assets? (not really a remake in fact, but not my own IP)

Yes, though in cases where IP might be restricted make sure licensing/permission permit it or the game is sufficiently changed to not infringe.

2- Can I reuse assets from an older game I made on a different platform and remake it on game boy? (100% new code, many reused assets, my own IP)

Yes, just make note of what pre-made (unchanged) assets are being used. See this part of the rules:

Pre-made and AI generated assets for art and music (and covers of other music) are allowed if the licensing permits. Be sure to make note of what is being reused or re-interpreted and under what terms. There is a specific field in the submission form to allow you to disclose this. Use of pre-made and AI generated assets and content (vs wholly original and human created) may be factored in when judges assess points for relevant criteria (e.g. graphics, audio).

https://gbdev.io/gbcompo25.html#general-criteria

Also note the theme for the Jam will not be announced until it starts.