A one by one multiplayer watch where you have a pre set map (which is randomly chosen) with.
Booth player fight for the most survived rounds or points(put them as different modes Score mode:).
Round Bonus is only given to the faster Player and is higher the longer the other one is defending or normal system. Wave skip is not possible. I thought of an idea that while playing you will receive a tile for Towers (booth players the same one) which you can place wherever you want.
With resources or/and money you can maybe buy power ups.
Another use for resources could be to send enemies to destroy the opponent.
Maybe make the bases have completely random boosts booth have the same.
Every enemy comes out of every portal except bosses they are not good for multiplayer.

Maybe a Boss-Raid mode would be fun where multiple players play and defeat like every 5 or ten waves some bosses.

I thought of a Fortress mode: Where are most of the map are roads and every enemy move a randomised path down to the base
Where are some tiles that act as barricade, will receive one damage per second and share on healthbar.
If the baricade gets destroyed you lost.
The enemies won't die until they got shot by a Tower.
And maybe enemies walk on path tile.