Just the one feature makes for an unappealing sales pitch, at the same time getting the first premium mobile client (as far as I know) would seem to me to be the overriding concern.
Actually screw it, just the one killer feature with a noncommittal suggestion of future work and the civic understanding that funding development is a good in itself would be better than a poorly-thought through whimsical extra you may later regret
If you come up with a really good idea for extra features down the line, there will be plenty of new users to come who will see that value proposition at the perfect time for them. If you rush something now with muddied motivations you’ll kick yourself for committing to something you may yet regret.