Because all the cool kids blog about it these days..
- Follow the FHS (get rid of workarounds such as Optification)
- Remove Maemoisms and Scratchboxisms
- Follow freedesktop.org specifications (at least bugs 657, 1303, 3817, 3818, 3828, 3822, 3913, and 4766 according to this wiki page)
- Allow many different UI toolkits and languages (it's OK if you decide to provide official support for only a few of them)
- Make sure other UI toolkits integrate nicely (mostly covered by following wm-spec, icon-naming-spec and systemtray-spec and not inventing "MeeGo"-specific technologies and non-standard implementations)
- Make sure notifications work from other toolkits (libnotify-based vs. Qt-only notification widgets)
- In general, follow the ideas from the Mainstream Linux Aligment wiki page (the page is broken atm, but you'll be able to see the bug IDs)
If any of these specifications/standards do not meet the requirements, please work with freedesktop.org or other related institutions to get your extensions discussed, fixed and then integrated into the relevant specifications.
People will still develop mobile UIs specifically for whatever device will come along in the future, but porting from/to "mainstream" Desktop Linux should be as easy as possible.
There's a lot of awesome open source Linux/UNIX (GUI) software out there, it just needs some UI love to be usable on mobile devices. Don't make it harder than it should be.
(And with that I mean that it's easier to relayout a GTK+, FLTK, wxWidgets, Swing or Tk UI than to port everything to "the one true toolkit".)
This is especially true since open source developers usually develop in their free time, and porting an app to a new toolkit/environment is something that can't be done in two afternoons. Relayouting UIs can be done in that time.
Adding paradigm-shifting cool new UIs will still work without breaking backwards-compatibility and without restricting developers to one language/toolkit.
Oh, and Mer solves most of these issues by basing itself on a "standard" Desktop distro and only changing stuff that's really necessary.