I never really liked how the rank up system essentially made players who played more (or really players who leave their computers on overnight simming) objectively better than other accounts with a lower rank. A new account can have nearly next to no chance in a race with a highly ranked account, given the stark stat point difference.

This does raise some other concerns, though. Stat allocation is something that does have some worth gameplay-wise. Given that PR4 seems to want PR2 levels to be played in a legacy format (many requiring specific stat builds), I think if it is to still exist in PR4, there should be some means to know how other players' stats are allocated besides trying to calculate them based on how the other player moves (visually, or some read out on their stats, whatever). 

On the other side of things, if one is to remove the stat allocation system entirely in lieu of a more consistent moveset between racers, this would mean that legacy PR2 levels would either not be a thing, or be in some other "legacy" mode, which would mean more development overhead having to maintain two different games, essentially. I'm perfectly fine with PR4 not having PR2 levels; PR2 players can always play that game if they want to enjoy its levels. I'd rather just a new game entirely, not PR2 again!

I'm not outright advocating for the removal of the EXP system entirely, though, just the aspect of that offers an objective advantage for higher ranked players. An extrinsic reward for racing is good; it makes our monkey brains happy seeing a number go up from racing. To reward players for racing in an extrinsic manner, perhaps instead of EXP one could gain currency that could be used in a shop to buy parts or other cosmetics. Not microtransactions! An in-game shop! Just like the good old days!

I guess I would also like to say that I would want the EXP/Kong hats to not retain their purpose as seen in PR2. Hats should only offer an intrinsic gameplay benefit. Pretty much everyone in PR2 wears an EXP Hat because EXP is king.

Title says it all, pretty much. The ability to race with 8 or more people in the same race! I've noticed some people getting left out of races because they have 4 people, so this would feel less restricting. Ideally, there would be like 12 people per race, sort of like the modern Mario Kart games.

Blocks that move in a direction chosen by the player. Kind of like the Move Block in PR2, but more unique. Ideally, a way to connect more than one block together so they don't get separated. Maybe a way to change how fast the block moves. This would create some unique platforming sections! For example, a platform moving back and forth over a gap.

This is just an example image I found of how it might look:

Rather than the one-bit color view of PR2's, use something like PR3's system, borrowing the block colors to create a more illustrative minimap view of the level. 
Could be nice for making minimap art.

Akin to Mario Maker, have levels displayed in the lobby have a preview. There could both be a small little thumbnail showing a bit of the level and a full view of the level; the thumbnail could be determined by the user (like Mario Maker, perhaps based on the location of their last save, or some other system).

If a player is looking to get a good time on a level, it's good to be able to practice any section at any time. Unfortunately, there's no really easy way to do this, unless you made the level yourself and want to practice in the LE, and even then it's a bit cumbersome, having to rely on teleports and the slow restart button.

Practice mode should enable something like the following:
- a means to easily reset one's position anywhere anytime, with a command/button press, for nailing hard tricks
- the ability to "noclip" and fly to any given point on the level, quickly and easily (could coexist with the usual LE click-to-teleport or, or use something like PR2's spectator mode, and it would be nice if scrolling the camera would be faster too) 
- pausing/changing the timer so one can practice for as long as they'd like

This could include a suite of other things; whatever's needed to give players all the tools they need to succeed in their runs. I'm sure other people could think of more things. This also would go hand in hand with my Retry button suggestion.

Clearing a level in practice mode would obviously not grant any of the rewards (medals, best time, EXP, what have you).

If a player is doing a single player race, there should be a seamless, easy "retry" button when playing a level that resets the level to the beginning without having to redraw/rebuild it, setting blocks and the timer to how they were at the beginning. I suppose this could also be an option in multiplayer; perhaps an option to vote for a reset, and if all players participating agree to it, it would reset. This should be as instantaneous as possible to reduce any downtime between attempts; within the frame, if possible. This could be a GUI button or a bindable key. This should also skip the countdown, or have it go instantly.
It's a real pain in PR2 resetting (say if one is looking to do a good speedrun), having to exit out of the level back into the lobby, rentering the level, and having it draw again (especially in artistically vibrant levels; disabling the art isn't fun!).

Rather than only having a fixed amount of art layers ala PR2, it would be nice if players could have as many as needed (or at least a good number more than what is available), both in front and behind the level's geometry. What would also be good is a fixed background layer that doesn't scroll. 
I believe PR3R has this feature.

Akin to PR3's campaign, levels should have custom par times players could shoot for; perhaps having just one par time or a gold/silver/bronze medal system. If a user sets medal times, there should be a clear check to make sure that clearing the level in that time is possible. If the level ends up in a campaign, admins could tweak the times if need be. Levels could also have saved world records, to challenge the best of players.

'nuff said. 45 degrees only could be good, but you could have a variety of different grades. 

It would make sense for the only blocks to be "slopified" are the basic blocks. Might have to change the way tiles are handled to make it look more clean.

Bonus points if you include a Mario-esque buttslide mechanic. This might be too much, though; how the game might improve on the movement is up to Jiggmin.