UI certainly needs some love - imo, it is one of Stardock's biggest failings recently.
Tooltips should appear ON MAP for multi-unit attacks / spells (probably even for single target). Looking in the corner isn't as convenient, and doesn't help at all for multiple units.
Tooltips on enemy units major abilities/hp/etc.
Ability to equip armor and see stats at same time. Ability to see all my hero abilities.
Notifications should appear in bottom right, not top left. Notifications need an X button to close when opened, not just minimize and then close.
Cities need growth and essence indicators.
Easier access to sovereign tree
Crafting UI needs a rework