While Excel has been fabulously successful, that is not the way I 
would go for a Tweak/Squeak/Sophie spreadsheet. Instead my first 
priority would be to have the best table layout possible since more 
people seem to use Excel for that than for actual calculations (not 
counting simple sorting of tables). And having several smaller tables 
floating around in a larger document seems nicer to me than a single 
huge 2D (almost 3D with tabbed "pages") array, though I can understand 
how such a structured space can make some people feel more secure 
(probably more veteran users than newbies).

For the second step it would be interesting to look at The Analyst for 
inspiration and allow arbitrary Smalltalk objects and expressions 
instead of just numbers and strings.

Third: I would separate the expressions from the actual cells in the 
style of Lotus Improv. This would encourage the users to think in 
terms of collections of data like in APL or FScript instead of 
individual numbers. It would also make doing things across more than 
one table less awkward. The user interface could still present things 
in terms of per cell equations when that is more convenient.

Lastly, having such a component I would use it in Sophie for things 
like timelines instead of hand coding stuff in Tweak.

-- Jecel