I'm sorry but with the exception of #1 I don't see these suggestions adding significant value. Nor would I expect them be integrated into a board intended to appeal to a large audience.
1> A way to burn the boot loader on a blank ATMega .
This would be interesting if the on-board 16u2 could be jumpered or something. However, I'm not sure the added complexity is worth it.
2> Several seven segments on board for experimenting and debugging .
I don't think this would appeal to a wide audience. Serial prints are far more effective and add no parts cost to the board. 7-segments aren't cheap in terms of cost or hardware resources. How would they be connected? Lots of trade odds for a feature few people have requested.
3> all needed for shield connections.
4> Zif socket
I think the size and cost of a ZIF socket isn't worth it. In the 3+ years I've been working on Arduino projects, I've only changed the chip a couple of times. Depending on volume, I have seen ZIF sockets that cost more than the current Uno. It isn't a small up charge.
5> Several mini potentiometers for analog inputs .
Again, this adds cost and complexity while only appealing to a small number of users.