Might be difficult, but if not, I would really enjoy seeing castles in there, a bit like the ones in Kanazawa where it's all spaced out not like himeji or Osaka where it's just a whole cluster pile. It would be cool if like your units could mount the walls or capture a fortification with the units inside able to change from player to player as long as they were able to maintain a certain area for a while (bit like King of the hill) also just as an additive canons would be a game changer for campers (guilty)
Also I reckon there should be historical battles in the practice mode, like replays of battles between clans. There was a heaps cool battle at sekigahara which would be fun to recreate.
Also smaller fortifications created by the units for example like hay bales or ditches or spikes(which would take time) for currency which (could) be earned in tournaments.
Also like numerous other people have mentioned here, the customisation of armies would be amazing and would provide a sense of character and give your army unique tactics. But there is still the problem of new players being destroyed by veterans if this goes through so it should be like a limit of units or like stronger units take up more space. So say for example the enemy could have 1000 peasants and you could have 700 samurai and it would be even(I don't know exactly there would have to be like ratios or something) and also abilities as well like fire arrows or formations etc.
Also I just thought of this now, but what if you created like a whole map of say a 100 or so castles owned by frequent players and new players coming in would be given a handicap also each of those castles would be a single Map to play on. Though I find this one a bit far fetched it would make a great campaign
Also I think it would be cool to have like a time thing where there are the four seasons which have different weather, so summers green autumns orange winters snow(which hinders movement) and then there's Spring which is pink (cherry blossoms) also a day and night would be cool and the fog of war idea someone had would be further enhanced with rain. Also from my idea earlier the fire arrows would create fire in the autumn which could burn down tree cover and injure soldiers
Speaking of which there should be like a thing where soldiers who previously engaged in combat and are injured could move slower or shoot less accurate but only for individual soldiers though.
Also the player credited with the most multiplayer wins should be literally crowned shogun.
Also if possible it would be cool if some of the rivers in the maps were flowing so a few soldiers get washed away and killed.
Also in some games increase the playable numbers so like 3v3 or 4v4 I don't know if this would be too difficult and would override the system.
Map selection would be cool
And how about with gunners you add like a thing oda nobunanga did with his gunners where they had them firing in rows of three so it was quicker but did less damage
Also it would be cool if you added a captain to each unit so when he died the unit routed for a short length or just lost a bit of moral or maybe like just add a general to the star horse unit and when he died all routing enemy units stopped routing
Also it would be cool if I could zoom in all the way and get a view of the battle from one of my soldiers eyes or get level so I can see the terrain and find a good spot for archers
Also combining units of the same type so they're easier to control so 2 katana units of 40 each join together to form 1 katana unit of 80
That's all I can come up with at the moment, even one of these ideas being put to use would be awesome. Keep up the good work.