People joining match that has already started (via Quickplay) must first land onto some screen where they can clearly see the mission’s name / objectives, its modifiers, current team’s composition and status (see everybody’s toughness / hp) - and be able to quickly change their build if needed, to adapt. Should be given 10-15s to make their choice. While choosing they should be able to spectate the match from the “eyes” of the bot they’ll spawn into after that.
To prevent abuses of the feature, it must only be allowed once per player, for this match - and only for players who join it in progress.