Hello! I’m an old fan player of Warcraft 3 ROC and TFT.
This is my first computer game and my favorite. I consider it a high standard of all RTS games. I love its mechanics and responsiveness, graphics of course.
This is my highly, even extremely important suggestion:
Relying on the experience of StarCraft 2 I hope you will not make the game absolutely dependent on servers. What do I mean? Here I will make a small digression: I love a series of StarCraft, but I have fully killed desire to play in second part of, because even in singleplayer present ping, when I want play with bots (AI) (I have weak Internet, but a powerful computer), ping taints all impression from game. I really hope that in Warcraft 3 Reforged this will not happen again, that just will be good old skirmish (option/button/submenu) in the game menu.
Also, I really hope that the old, my favorite, I think it is the best game world editor, will not turn into an inconvenient, although much more functional editor in the likeness of StarCraft 2 world editor, and will receive its natural, evolutionary development.
I really want to play for fun, as it was in the good old days, in skirmish with the maps edited by me, without any ping. And many-many people I guess.
Obviously that even an almost perfect Warcraft 3 formula is outdated for decades and it is time to make major changes in the although successful, but outdated formula, but not so revolutionary that will lead to drastic changes in the mechanics of the game or any other aspects, and will scare away fans (like me), but the changes of an evolutionary character: add new race, but it is correct to talk - fractions, as all sides of the conflict are internally very heterogeneous. Very popular is now theme with Naga, and because nor in one of sides creatures this fraction there, and it there is as mini-fraction for campaign, its resources can be engage to create fully-fledged gaming fraction. And also develop the game - saturate it with content of a similar, global nature! I believe that it would be very reasonable and effective both for the development of the game, increasing revenues, and for the community. (increase of interest, etc.)
I’ll add on the little things: But the devil is in the details!
My experience of original Warcraft 3 build on excellent unit animations, smooth and high quality. This is an extremely important thing for the perception of what is happening in general. It is also important as well as bright juicy colors and contrastive lighting! Also relying on the beta experience, I hope for a normal - smooth game menu, as it was in the original.
About UI - it is too old even now, then it supports widescreen. It takes up a lot of space and looks very bulky, in 2018 during the announcement of Warcraft 3 Reforged at the presentation was a wonderful fresh, modern and compact, beautiful interface, great for rejuvenated Warcraft, so let it be in Reforged. 
Otherwise it looks to put it mildly strange, as if a 100-year-old pensioner puts on youth clothes.
Otherwise it turns out that the classic version will be better than the new one.