Ok I think I forgot to mention a few things as people are jumping to conclusions. If I do use existing characters I will obtain permission/ License. So that is of no concern to me. Plus i never explicitly mentioned a company.
Then there has to be some advantages to using existing characters because publisher do it all the time. Besides the obvious established fanbase.
Depending on the type of game new sprite work might have to be done original or unoriginal etc...
As for the trademark im pretty sure "Ninja Frogger" as one entity is not trade marked but if it is i simplely change it. Should be No big deal Ninja Gaiden/Tecmo not filing against Fruit Ninja or Final Fight agaisnt Final Fantasy. Just examples. If a problem arise im sure a simple agreement can be made. Trademarks are registered by word/design.
Just an example using "Frogger" check Trademark Electronic Search System (TESS)for your nation.
I checked and Ninja Frogger is not registered atleast in the US(dont know how to check for other countries but US site does it matter) and other companies have used frogger in their name. Only problem same industry but still different platform.
Then the likelihood of publisher knowing about fanmade sprites and doing something about it is extremely low. To common and costly.
Egh, head ache i think ill stick with original for this game but i still want to know advantages cause I would like to talk with other people about porting games through agreement one day.