I actually just registered on this forum to leave my reply, because I have strong feelings about the proposed changes. I've been following DDs, but only now decided to chip in. Apologies if some of my ideas have already been spoken out.
1. I don't feel Doomstacks and current fleet engagements are such a big problem. Although I like idea about Admirals and capping amount of ships they can command, I feel a bit nervous about giving huge raw boosts to the smaller force against bigger. Its a bit unfair, to the bigger force. What I'm trying to say is that a larger force, even though it invested a lot of its resources into a fleet, they will still find it hard to win the battle/war against the enemy who didn't invest much into their fleet. So, at the end of the day its not worth investing that much into your fleets, because your smaller fleet will get an automatic boost against a bigger enemy.
Im not saying, a huge fleet should make war a piece of cake, but I just want to ask you be careful with the balance.
1.2. I do like idea of smaller forces being able to harass bigger fleets, so that a smaller / tall / technologically advanced empire has if not a definite, but still a chance to survive a defensive war against a bigger and more brute empire. Something like Guerilla tactics: traps, hit and run, raids etc. Your ideas are exploring this, but I think it'd be great if we could go a bit more in-depth into guerilla warfare. But Im not sure if simple raw bonuses to smaller fleets is the best idea. IMHO
2. Combat computers. Really good stuff! Finally, we should be able to tell our long-range artillery battleships to keep their distance and corvettes perform hit and run. Just on that note, it'd be great if you could implement features from 1.2. - so that corvettes could perform "guerilla-like" raids on large fleets to harass ships shields/armour/hall, without sustaining massive damage. Again, might be balance breaking or people might find it boring after a while, but I do think that Stellaris would benefit from more guerilla warfare.
3. Weapons - good stuff. Even more specialisation of the ships is great, but that creates a huge need to be able to see what weapons your potential enemies use. Are there any plans for any espionage mechanics, to be able to recon enemy ship design? I know that you can see enemy ships if you agree to trade sensor links, but I dont know why but my sensors can never make sense of enemy ship details. Spies would be a really cool addition.
Also, now that I'm here, I'll allow myself to write out some features that I'd like to see in future updates. Feel free to skip if you're not interested :0
1. Mercenaries. There was a mod for that but it was never flashed out. Rich, but small/tall empires could hire mercs to defend their worlds or allow a player to control a merc fleets inside it own territory. Or simply being able to rent a huge fleet for a fixed amount of time, but for a high price for your dirty work.
2. Espionage mechanic. Touched on that, but it'd be great if you could cause civil unrest at different empires through factions; steal technologies like in Civ; cause terror attacks on buildings/fleets; assassinations of the admirals/kings. All of that but with counter-espionage mechanics, which would be quite expensive.
3. Ground warfare. Im pretty sure people have spoken about that, but we need to tackle somehow the fact that defenders on a planet are always at disadvantage. They are hard capped at the amount of units they can keep, but attackers can bring an unlimited number of units and swarm the limited defenders. I dont know, maybe you did it on purpose, but I think offensive wars (especially ground ones) should be more expensive/hard than defensive.
Oh, and can you please also add templates for the ground units. Its a pain in a neck to add attachments to every single unit.
4. Ship trading. Im pretty sure one of the devs said that it'd be hard to balance it right, but still. It'd be great if you could export/sell your ships to other empires for cash (or whatever you ask in return). Its an additional source of income and allows to increase speed of production if an empire is at war. Even if their technology level is below, they should be able to use the advanced technology (with a certain penalty and not learning technology). It would work both ways - you're at war and you need ships ASAP - pay some minerals to your allies and they'll start producing advanced ships and give it to you. It'll be expensive, but hey - thats the Switzerland way
Whew, it was surely a long post. Thanks if you read it. Im pretty confident that most of my points have been said already, but I still want to get my opinion onto here, cause I really enjoy Stellaris and I think Devs are doing great job!
Thanks!