At my day job, we are currently porting our portal to Sharepoint MOSS 2007. Like it or not, there is alot of big companies following suit. After working with the consulting company that setup ours (and buying their EXPENSIVE crappy webparts), I wanted to know some annoyances in MOSS 2007 that would make good commercial webparts. Or would it be a better to focus on making webparts for 2010? (as some of the issues in 2007 might have been solved there)
Sharepoint 2010 will be different from 2007 and would require you to update your skillsets. 2010 will only used by those who are setting up a fresh farm, coz the main hindrance for the success to Sharepoint 2010 is that businesses need to update their hardware. Those who already have 2007 would atleast continue using the same untill 2015 i guess. So either ways, you have a market to sell.
I agree with what Colin said that you will never have anyone giving you ideas that can mint good money. I am not a programmer, but I can still generate an idea. The point again is, why would I put those ideas on a public forum! :P
I can give you a tip however. What ever you find as difficult to achive in a normal scenario can be sold. You can go to various vendors who are selling Webparts and also take inspiration from their creations. Make sure you read the fineprint and avoid replicating something that is patented or an invention.