I've put quite a bit of thought into what I think SMF 2.1 should have, should not have, and what should be modified, so I thought I might as well share these thoughts with the public.
In my opinion, SMF should just support MySQL, or have the option to select their database type on the installation.
All old browser support should be dropped, such as IE6. There are quite a few limitations supporting old browsers cause, and there has to be some point where developers can develop Web 2.0 code without worrying about old browser compatibility and having to code a lot of extra lines to keep it compatible with ancient browsers.
CSS3 & jQuery Implementation:
jQuery is most definitely in the future of the web, it is highly extensible and at the same time lightweight. If for no other reason, it should be included on a boolean variable for mod authors to enable if their mod uses it.
In my opinion, the default theme should also contain some nice attributes that CSS3 has to offer.
Most modern browsers contain some form of spell-check built in, or an addon for it. SMF's also requires a third party script to run. So in my opinion, it's not even practical.
Although I do believe it could be updated, and provided as an official addon/modification.
I believe the Calendar should not be a core-feature. I think it should be removed from the SMF Core, and completely re-written to have an AJAX/jQuery fancy interface, provided as an official addon/modification.
$smcFunc & PDO:
$smcFunc should be re-written as a PHP Object for easier usage, and a much shorter line-count to accomplish a much better result. Plus some more automation such as auto-data type with the ability to over-ride? For instance, with $smcFunc in 2.0, you have to declare each columns data type, such as 'field' => 'int', and so forth, but this can be figured out with PHP very easily, with no need to declare each column.
Along with the PHP Object, PDO should be implemented for data sanitation.
File Based - Database Modifying - Menu:
There were huge benefits with the move of the menu from template files in 1.1, to Source side in 2.0, however this still could be much more customizable for forum admins who do not know any coding, and even for those who do, much more simple and quick.
The solution is a file-based, and database driven modifying for the menu, with a very customizable and easy to use interface in the admin panel. (Menu Editor Lite + some features of full)
It wouldn't be bad to consider the same idea of "Menu Editor" for Bulletin board code, such as the ability to remove bbc items, or add custom ones, permission to use one, and so forth, through an easy interface, outputted to a file so there wouldn't be any extra queries on a regular page load.
Bloated Profile Fields:
I believe bloated profile fields as "MSN" - "ICQ" - "AIM" - "YIM" should be removed. With the addition of Custom Profile Fields in SMF 2.0, why provide some fields, but by no means them all? What about Skype, Google Talk, Facebook? I think it's much better to remove all un-necessary fields but allow them to be added easily through Custom Profile Fields. You could even have "Custom Profile Field Templates".
The memberlist should be configurable through the admin panel, with different two different listing styles. Block / List.
The buddy list is very limiting, and practically useless. This should be moved to an actual database table, and the features expanded.
The profile sections of SMF are lacking, and very behind today's technology and social aspects. Inline editing, more social such as visitor messages / comments / notifications / etc.
Recaptcha to replace SMF's default captcha?
Personal Messages should definitely have the ability to include attachments.
Quick Reply vs. Posting Page:
The possibility of completely removing the actual "post" page and making something similar to Quick Reply be the main form of posting, with BBC editor obviously, and attachments. (Attachments powered by AJAX?)
News / Newsletter:
Provide a more intuitive Newsletter system, with a more advanced news system being an official addon/modification.
Who doesn't have a smartphone today? Provide jQuery Mobile for smartphones. "Posted from iPhone" or "Posted from Android" (very simple to addon, but a neat idea imo)
Email Template Editor:
Provide a simple but customizable interface to modify email templates? Probably better kept out of core, and added as an official modification.
External Links / Blacklist:
The ability to "blacklist" an address, when the link is clicked, warn the user that the link is blacklisted, ability to proceed or stop. Let users know that they are clicking a link that is external. Enable or disable...possibly a modification rather than core.
Users Online Today:
This should definitely be a core feature by now.
Remove all of the old .gif's that are old fashioned looking, and replace with a nice new icon pack.
I have more thoughts, but I think this enough for one post.
Definitely feel free to comment or question any of my ideas.