I've given this a bit of thought after the games with Rampoina. For the record, we played 3 maps:
- Uphill battle. Rampoina was too strong for this map, and I didn't stand a chance. So we moved to
- Ivory Tower: We played quite a few rounds, Rampoina won one and came very close to winning 3-4 times.
- Stepping Stones: I handicapped myself by trapping my crowd and waiting for 1 minute before acting. Within 1 minute, building from the bottom middle block can almost reach to the opponent's starting platform, so this made it quite intense when my crowd was trapped there. If the spawn was simply delayed, the stakes would be lower, and even a delay of more than 1 minute could be considered. Rampoina almost won one round via the top route, but the crowd splatted right in front of the exit x_x
I believe handicap is extremely map specific. Quite often a delay and the reduction of the number of lix can serve the purpose. But many other maps will have specific skills which are rare, and adjusting their number might be a better means of handicap (e.g. floaters in Rescue Rangers, or diggers in Pancake Compression). The problem is, finding a good handicap needs knowledge of the map, and presumably the person who knows the map best is the level author.
I don't think putting the burden on the players via some complicated UI allowing to adjust delays, skills, number of lix etc is necessarily the best idea (it might be useful for experimenting though).
I would argue the best way would be to introduce handicap levels (both positive and negative), and the level author can specify various handicap levels in terms of delay, number of lix, spawn interval, skill numbers, etc.
Intermediate levels of handicap that are not specified would be done via linear interpolation (or maybe logarithmic for skills and/or lix amount, not sure). Of course, getting these levels consistent across different maps is difficult (and also subjective), but easy to tweak by changing the specific handicap level that is associated to a specific skill/lix parameter tweak.
I'm advocating for both positive and negative handicap because for some maps, it might be easier to provide positive handicap (e.g. add blockers, or add floaters to Rescue Rangers, while you wouldn't want to make these adjustments to the level itself). The aim would be that the handicap difference indicates the advantage, so +10 and 0 would be similar to 0 and -10.
Another advantage is that you can assign handicap values to spawns in asymmetric maps (and then possibly adjust them further via lix/skill parameter tweaks, to allow for a range of handicaps), incorporating these maps straightforwardly into the system.
From a player perspective, one would then simply adjust one's handicap level in the multiplayer UI, without needing knowledge of the specific map. Not every map would offer handicap levels, or some might only offer a limited range; the levels might not be 100% consistent across maps but one could make reasonable efforts towards that. I'd argue that all of this is fine, for example, the level browser could filter to only show maps where the min/max handicap is larger than a user provided value.