I have been working on some improvements on the Relgraph visualization over the last couple of years for my own tree export. I think it is quite nice and may benefit others, but integrating it into the official releases myself is way beyond the time available to me.
What I basically added to the relgraph are features to highlight the path of ancesters for the center person in trees that include other people as well and also embedding images for persons, events and locations which makes the resulting tree much more fun to read.
It looks different enough from the standard relationship graph that I would strongly urge you to make it an addon with a different name. We can approve and release addons much more quickly than changes to gramps core plugins.
Make a new .gpr.py file for it, changing the name of the addon etc., create a personal branch off of the gramps-addons github, and submit it as a PR. Lots of examples in the gramps-addons to help with this. You can make a subdir to your gramps51/plugins directory with the gpr and source files, to test on your own machine. It should end up added to the graphs menu in Gramps and work if you did it right.
There is a preliminary wiki page (Adapt a built-in Report) which talks about duplicating a 5.1 built-in as an Add-on for redevelopment.
This might be a good opportunity to point out where this page needs to be expanded.
I went through a similar process for posting a variant of a built-in Gramplet as an Addon. It required copying the .gpr.py info from the combined one. (There is one for each type of plugins, including: reports, graphs, tools and gramplets.)