Hi Antoine,
Thanks for your interest in developing the Pluto frontend! I added the current Pluto frontend a while back (with support for Chombo-style HDF5 files only), but I don't think it was ever really used and has likely fallen into disrepair as yt has evolved from 2.X to 3.0. It would be awesome if you'd like to resurrect it and make it into a fully-supported piece of yt.
Currently, there is a lot of code duplication between the pluto and chombo frontends. If you are working with Chombo HDF5 outputs, it's likely that yt 3.0 can already load your datasets via the chombo frontend, but it won't know things like what field names to expect in your dataset, or be able to set up units properly. To add support for these features, you probably want to sub-class the ChomboHierarchy, ChomboDataset, and ChomboFieldInfo classes found in frontends/chombo/data_structures.py and frontends/chombo/fields.py. I'd be more than happy to help with this, if you want.
If you're not working with the Chombo-style outputs, then I'm afraid I won't be much help, and you'll have to create a new frontend from scratch.
At any rate, you should generally feel free to go to town on any of the code in frontends/pluto, as I don't think anyone is relying on it.
-Andrew